Add Registry-wide unknown-flag probe (mix bash_fixtures.gen flags)

[davydog187]

Implements #70 item 2 for the Registry-wide unknown-flag probe only: a generated flags_matrix suite in the same generate / record / test path as date_matrix, printf_matrix, test_matrix, and varop_matrix.

This is not the item-3 filesystem-shape × command cube, Oils activation, or CI topology.

unknown_flags_test.exs from #68 keeps the FlagParser unit tests (exact GNU wording, clusters, --help). The registry-wide classification table moves here so there is one source of truth.

What was enumerated

mix bash_fixtures.gen flags writes 184 cases (92 Commands.Registry names × {--jb-not-a-flag, -Z}). A new Registry command fails generation and the gen test until it is classified with a reason — the same completeness rule as end_of_options_test.exs.

• Every name is generated. Operand commands (echo, test, true, :, false), GNU -Z acceptors (ls/cp/mkdir/mv/id SELinux, diff -Z, grep -Z, curl -Z, file -Z), loop/numeric builtins, and JustBash-only markdown/md are not omitted.
• ls -Z is recorded against an empty workdir so the listing is not the workspace.
• Scripts end in echo rc=$? so a wrong exit code is a stdout difference too.

mix bash_fixtures.gen flags --dry-run reports flags_matrix: 184 cases. mix bash_fixtures.gen now generates date, printf, test, varop, and flags.

Recording

Docker was not available on the agent VM. Oracle output was recorded with the same test/fixtures/runner.sh the Docker task runs, under GNU bash 5.2.21 / Ubuntu 24.04 (uid ubuntu, not root), then pretty-printed through Mix.Tasks.BashFixtures.write_json!/2:

mix bash_fixtures.gen flags
bash test/fixtures/runner.sh < test/fixtures/bash_cases/flags_matrix.json > /tmp/flags_matrix_raw.json
# then Jason-pretty-print + Fixtures.validate, as `mix bash_fixtures` does

Host extras used for twins the Docker runner image does not ship: tree 2.1.1 and xxd (vim 9.1). To re-record through Docker when it is available (those two will become command not found unless the image grows them):

mix bash_fixtures flags_matrix

Known gaps (161 of 184)

No cell was omitted. Divergences are opts.known_gap (excluded from the digest). The fixture runner still executes them with the assertion inverted.

23 cells match bash unmarked: :, echo, false, test, true (both flags); FlagParser exact GNU wording on sort/head/tail/tr/uniq (both), cp/grep/ls (--jb-not-a-flag only).

Count	Reason
48	Misblamed as a filename or other operand; bash names the option
40	Both refuse; wording or exit code differs (Try --help, bash: line 1:, env 125 vs 1)
38	Absorbed as operand/format at exit 0; bash refuses
6	Non-numeric status/count (exit/return/shift) not diagnosed the way bash does
6	local/declare/typeset parse flags outside a function
5	GNU -Z is SELinux context (ls/cp/mkdir/mv/id)
4	GNU -Z is a real flag (diff/grep/curl/file)
4	break/continue omit bash's "only meaningful in a loop" warning
4	markdown/md are JustBash-only; bash reports command not found
2	[ without ] still evaluates; bash exits 2
2	read is quiet; bash writes usage
2	yes repeats the flag; GNU yes refuses unknown flags

JustBash.exec/2 did not raise on any cell. No remaining unknown-flag implementation bugs were fixed in this PR — it is the enumeration + harness.

Cheap follow-ups the matrix now makes un-skippable

• Migrate absorbed parse_args commands onto FlagParser (Unknown flags are silently absorbed as operands, so sort -Q and uniq -Z return empty output at exit 0 #68)
• Name the option instead of a filename on misblamed commands (cat, rm, wc, …)
• Align refusal wording with GNU (unrecognized option, Try 'cmd --help')
• yes should refuse unknown flags instead of repeating them
• [ without ] should exit 2

Not in this PR: the item-3 FS-shape × command cube, Oils activation, CI topology changes.

Tests

All local quality gates passed:

• mix compile --warnings-as-errors
• mix format --check-formatted
• mix credo --strict (only the existing intentional banned_fixture_apply finding)
• mix dialyzer (zero new errors; 13 skipped, same as main)
• mix test — 2 doctests, 62 properties, 6810 tests, 0 failures
• mix test --only suite:flags_matrix — 184 tests, 0 failures
• mix bash_fixtures.gen flags --dry-run — flags_matrix: 184 cases

Related Issues

Related to #70 (item 2, Registry-wide unknown-flag probe only; not closing the issue)

Type of Change

• New feature (non-breaking change which adds functionality)

Testing

• Added new tests
• All existing tests pass
• Tested manually (mix bash_fixtures.gen flags --dry-run, local runner.sh recording, JustBash vs oracle classification)

Checklist

• My code follows the style guidelines of this project
• I have run mix format
• I have run mix credo and addressed any issues
• I have added tests that prove my fix is effective or that my feature works
• New and existing tests pass locally with my changes
• I have updated the documentation accordingly
• I have updated the CHANGELOG.md (for non-trivial changes)