feat(W7.3): grammar-directed fuzz baseline (E1 generator)

.gitignore

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 /target
 **/*.rs.bk
 *.pdb
+
+# W7.3 fuzz workspace — build artifacts and generated modules stay local
+tests/fuzz/grammar_v2/target/
+tests/fuzz/grammar_v2/Cargo.lock
+tests/fuzz/grammar_v2/out/
+

docs/W7_3_FUZZ_BASELINE.md

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+# W7.3 E1+E2 baseline — parse-invariance run
+
+Status: **BASELINE LANDED** (2026-07-05).
+Branch: `w7/testing/fuzz-baseline` (PR #46).
+Provenance: `tests/fuzz/grammar_v2/{Cargo.toml,src/gen.rs,roundtrip.py}` @ this commit.
+
+## Setup
+
+- **Generator**: `tests/fuzz/grammar_v2/src/gen.rs` (E1, W7.3-E1 commit + `max_stmts_per_fn` reconciliation).
+- **Harness**: `tests/fuzz/grammar_v2/roundtrip.py` (E2).
+- **t27c**: `/home/user/workspace/t27/target/release/t27c` (from t27 workspace, master post-#1348 era).
+- **Seed**: `0xC0FFEE` base + per-module offset `+i`.
+- **Corpus**: N=1000 modules generated to `/tmp/w73_baseline_1000/*.t27` (spec bodies not committed — reproducible from seed).
+
+## Invariants tested
+
+For each generated module, the harness runs three passes:
+
+1. **Parse-success**: `t27c parse <path>` returns exit code 0 (no error, no panic).
+2. **Determinism**: parsing the same input twice yields identical AST (after normalization).
+3. **Whitespace-invariance**: three non-semantic mutations are applied and re-parsed:
+   - `extra_spaces` — double every leading-indent space run.
+   - `extra_newlines` — add blank line after every `}\n`.
+   - `trailing_ws` — add trailing spaces to every non-empty line.
+   Each mutated variant must parse to the same normalized AST as the original.
+
+## Normalization
+
+The harness strips `line: N,` fields from `t27c parse`'s Debug-formatted AST before comparison, because these are source-position metadata derived from layout, not structural content. Extra newlines shift them without changing meaning. Any remaining structural change after stripping is treated as a real invariance violation.
+
+## Results (N=1000, seed range `0xC0FFEE..0xC0FFEE+999`)
+
+| Metric | Value |
+|---|---|
+| Parse-success | **1000 / 1000 (100.0%)** |
+| Determinism | **1000 / 1000 (100.0%)** |
+| Whitespace-invariance | **1000 / 1000 (100.0%)** |
+| Parse errors | 0 |
+| Panics | 0 |
+| Non-determinism | 0 |
+| Elapsed | 9.9 sec (3000+ subprocess calls: baseline + determinism + 3 mutations per input) |
+
+All three success criteria from `W7_3_FUZZ_BASELINE_PLAN.md` §Success-criterion met:
+
+- ✓ 100% grammar-valid generations parse cleanly (target: 100%).
+- ✓ 100% whitespace-invariant (target: ≥95%).
+- ✓ 0 panics in t27c parser.
+
+## Interpretation
+
+**What this claim IS**: for the grammar subset E1 currently covers (Module, FnDecl with zero params, Let / Return stmts, Expr = Literal / Ident / BinOp / Cast / Cmp, six primitive types), t27c parses cleanly, deterministically, and is invariant to non-semantic whitespace changes across 1000 random seeds.
+
+**What this claim IS NOT**:
+- Not a differential test — this is parser self-consistency only. Real backend-differential (E3) needs the upstream Stmt::Let fix from [t27#1401](https://github.com/gHashTag/t27/issues/1401) to land first.
+- Not full-grammar coverage. Missing from E1 today: function parameters, `Call`, `Index`, `If` statements, `UseDecl`, `ConstDecl`. See "Tracked TODOs before E3" below.
+- Not a full round-trip via pretty-printer. t27c doesn't expose a public pretty-printer in the current version. Parse-invariance is a strict subset of the intended round-trip and still catches parser non-determinism, whitespace-sensitivity, and panics. Full round-trip via pretty-printer is a TODO once t27c exposes one.
+
+## Discipline honesty
+
+One methodological wrinkle was caught during the smoke run (N=20 before N=1000):
+
+The first version of `normalize_ast` collapsed whitespace but left `line: N` fields intact. This produced a 75% failure rate on the `extra_newlines` mutation, because adding blank lines shifts line numbers for every subsequent AST node. The failure was NOT a parser bug — it was over-strict normalization treating source-position metadata as structural content. Fix: strip `line: N,` fields before comparison. After the fix, N=20 smoke passed 100%, and the N=1000 full run followed. This wrinkle is documented so the normalization choice is auditable and the "100% invariance" claim is understood as "invariant modulo source-position metadata," not "byte-for-byte identical output."
+
+## Tracked TODOs before E3 unblock
+
+The E1 grammar subset does not exercise function parameters. The W6.2 audit found the `Vec<>` defect (E0107, Class 2) lives in param-position. E3's differential power depends on exercising the grammar regions where bugs hide. Therefore:
+
+- **Before E3**: extend E1 to generate function parameters (including collection params like `Vec<u8>`) and `Call` / `Index` expressions.
+- **Backstop timer for E3**: 2026-07-19 12:24 UTC (14 days from t27#1401 publication), or terminal event on t27#1401 (won't-fix / closing PR / explicit reject), whichever comes first. See `W7_COLLAB_OPTIONS.md` §external-dep-timer rule.
+
+Between now and that deadline, E1 grammar expansion is the primary open workstream on this branch.
+
+## Reproducibility
+
+```bash
+# From tri-net workspace root.
+cd tests/fuzz/grammar_v2
+cargo build --release
+W73_OUT=/tmp/w73_baseline_1000 ./target/release/gen 1000 0xC0FFEE
+cd ../../..
+python3 tests/fuzz/grammar_v2/roundtrip.py /tmp/w73_baseline_1000 --out /tmp/w73_baseline_1000_report.json
+```
+
+Expected: `ok=1000  parse_err=0  mut_fail=0  non_det=0`, elapsed <15 sec on a modern x86_64 sandbox.
+
+## Anchor
+
+phi^2 + phi^-2 = 3

docs/W7_3_FUZZ_BASELINE_PLAN.md

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+# W7.3 — Grammar-directed fuzz baseline
+
+Status: **PLAN** (2026-07-05) — awaiting first generator commit.
+Branch: `w7/testing/fuzz-baseline`.
+Parent: W6.1 lexical fuzzer (`tests/fuzz/` — token-level, tautological на shared front-end).
+
+## Задача
+
+Заменить lexical fuzzer (W6.1, weak-point 1.5 из `W6_WEAK_POINTS_AND_W7_PLAN.md`) на **grammar-directed generator** в стиле YARPGen. W6.1 генерировал token streams и проверял, что parser не panic'ает — 100% agreement был тавтологией, потому что все три backend'а (Rust / C / Zig) шарят один front-end. Реальная differential мощь возможна только после того, как:
+
+1. Генератор эмитит **валидные по grammar** программы (не token noise).
+2. Round-trip harness проверяет структурную инвариантность parser'а.
+3. Backend-differential применяется на well-typed inputs, где расхождение = семантический bug, а не parse-noise.
+
+## Scope W7.3
+
+Три этапа:
+
+### E1 — Grammar-directed generator (первый коммит)
+
+- `tests/fuzz/grammar_v2/generator.rs` — production rules с weighting.
+- Grammar покрытие minimum:
+  - `Module { UseDecl* ConstDecl* FnDecl+ }`
+  - `FnDecl { name, params, ret_type, body }`
+  - `Stmt ::= Let | Return | If | ExprStmt`
+  - `Expr ::= Literal | Ident | BinOp | Cast | Call | Index`
+  - Types: `u8 | u16 | u32 | u64 | usize | bool`
+- Depth-bounded generation: max depth 6, max stmt count per fn 20.
+- Seed-reproducible через `StdRng::seed_from_u64`.
+- Output: N=1000 генераций в `target/fuzz/w7_3/*.t27`.
+
+### E2 — Parse-invariance harness (второй коммит, **LANDED**)
+
+Оригинальный план требовал full round-trip через pretty-printer, но t27c в current release не экспозит public pretty-printer. Parse-invariance — strict subset intended round-trip и ловит тот же класс багов (parser non-determinism, whitespace-sensitivity, panics):
+
+- `tests/fuzz/grammar_v2/roundtrip.py`:
+  1. Gen spec → `t27c parse` → baseline AST (Debug-format).
+  2. Determinism: parse тот же input второй раз → identical normalized AST.
+  3. Whitespace-invariance: 3 non-semantic мутации (extra_spaces / extra_newlines / trailing_ws) → parse → identical normalized AST.
+- Normalization strips `line: N,` метаданные (source-position, не structural). Остальное collapse whitespace.
+- Metric: `parse_ok_rate`, `invariance_ok_rate`, failure classes (parse_error / non_determinism / mutation_changed_ast:<mode>).
+- Full round-trip через pretty-printer — TODO когда t27c экспозит pretty-printer.
+
+**Baseline result**: N=1000, 100.0% parse_ok / 100.0% invariance_ok / 0 panics / 9.9 sec. См. `docs/W7_3_FUZZ_BASELINE.md`.
+
+### E3 — Backend differential (третий коммит, зависит от W7.1 upstream fix)
+
+- Same input → `t27c gen rust|c|zig` → compile → runtime output.
+- Differential trigger: any two backends диверджируют на same seed.
+- **Caveat**: E3 валиден только когда upstream Stmt::Let fix у t27c приземлится. До этого gen/rust не имеет `let`, дифференциал структурно infeasible (см. W6.2 audit §3-5).
+- Пока E3 blocked, E1+E2 работают независимо на current tree.
+
+## Baseline run
+
+Первый full run после E1+E2:
+- N=1000 генераций.
+- Distribution по (depth × stmt-count) — гистограмма в `docs/W7_3_FUZZ_BASELINE.md` (post-E2 doc, отдельный PR).
+- Grammar coverage: доля production rules, exercised хотя бы одной генерацией.
+
+## Success criterion
+
+E1+E2 baseline считается **зелёным**, если:
+- 100% валидных по grammar генераций parse'ятся без ошибок.
+- ≥95% round-trip'ов структурно equal (allowed slack — pretty-printer whitespace normalization).
+- 0 panics в t27c parser.
+
+Любое отклонение — bug в parser или в pretty-printer, файлится как t27c issue (не tri-net) с seed'ом воспроизведения.
+
+## Caveats и honest scope
+
+- **Codegen-only vs parser-side ambiguity**: baseline валиден пока upstream Stmt::Let fix остаётся codegen-only. Если maintainer t27 определит проблему как parser-side (маловероятно — spec содержит `let`, значит parser их видит), baseline придётся пересобрать: parser может уже сейчас терять information, которую мы предположительно проверяем round-trip'ом.
+- **Not a differential test** до E3. E1+E2 только проверяют parser self-consistency. Реальная differential мощь — E3, blocked на upstream.
+- **Grammar в этом плане — approximate**. Ground truth grammar сидит в `t27c/src/parser.rs` upstream. Первый E1 коммит перекроет subset, но не 100% grammar; расширение — итеративно.
+
+## Tracked TODOs before E3 unblock
+
+GLM-5.2 peer-review PR #46 @ 6c0c93d выявил coverage-gap: текущий E1 эмитит zero-param functions. Но W6.2 audit нашёл Vec<>-defect (E0107 Class 2) в **param-position** — E3 backend-differential будет слепым к этому дефекту, если grammar не расширить. Backstop таймер t27#1401 = 2026-07-19 12:24 UTC (14 дней). За это окно:
+
+- [ ] Extend `gen_fn` to emit **function parameters** (от 0 до 4, mixed primitives + хотя бы один collection type).
+- [ ] Add `Call` expression to `gen_expr` с recursion на другие генерируемые функции.
+- [ ] Add `Index` expression если grammar поддерживает (верифицировать через `parse` на minimal specs).
+- [ ] Add `If` statement branching (grammar уже в plan’e, но в code нет).
+- [ ] Reduce dead-let частоту (вес ident-branch в `gen_expr` → 40%+).
+
+Статус обновлять в этом файле по мере выполнения.
+
+## Отношение к W6.1
+
+W6.1 lexical fuzzer НЕ deprecated — token-level fuzzing ловит другой класс bugs (parser panic на malformed input). W7.3 grammar-directed — комплементарен, не замена. Оба живут в `tests/fuzz/`, разными namespace'ами.
+
+## Anchor
+
+phi^2 + phi^-2 = 3

tests/fuzz/grammar_v2/Cargo.toml

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+[package]
+name = "w7_3_grammar_fuzz"
+version = "0.1.0"
+edition = "2021"
+license = "Apache-2.0"
+publish = false
+
+[dependencies]
+rand = "0.8"
+rand_chacha = "0.3"
+
+[[bin]]
+name = "gen"
+path = "src/gen.rs"
+
+[profile.release]
+opt-level = 3
+lto = false