Add actionable compiler diagnostics

[konojunya]

背景

Stack consumers need compiler-owned correction guidance so Web and future editors can explain source errors without duplicating language semantics. The specification now requires ordered expected values and provides canonical actionable-diagnostic fixtures.

実装方法

• Add ordered expected values to the public diagnostic model with an empty default.
• Report exact parser expectations and corrective help at recursive-descent boundaries.
• Report all supported node kinds, edge kinds, and layout directions for invalid closed-set values.
• Suggest up to three nearby node or layout identifiers using Unicode-scalar Levenshtein distance, deterministic ordering, and related declaration ranges.
• Add corrective help to encoding, string, duplicate, constraint, and complexity diagnostics.
• Pin conformance to specification commit 7f9154d22702ddf02f2713bbc06dde7bdf635806 and compare expected only when a canonical fixture opts in.

テスト方法

• cargo +stable fmt --check
• cargo +stable test
• STACK_SPECIFICATION_DIR=../specification cargo +stable test --features conformance --test conformance
• cargo +stable clippy --all-targets --all-features -- -D warnings
• RUSTDOCFLAGS="-D warnings" cargo +stable doc --no-deps
• cargo +stable llvm-cov --lib --all-features --workspace --fail-under-lines 95 --fail-under-functions 95 --fail-under-regions 95
• cargo +1.85.0 test
• cargo +1.85.0 clippy --all-targets --all-features -- -D warnings

Review 視点 / 懸念点

The diagnostic representation is additive but public. Identifier suggestions intentionally use the specification threshold and never exceed three results. Human-readable wording is not a compatibility contract; structured expected values are exact only where conformance fixtures opt in.