lint: recurse W003 into unobserved blocks and match arms

[Nitjsefnie]

What does this PR do?

Fixes #782. check_func_unreachable recursed only through AST_IF/LOOP/FOR/FUNC/TRY/PROGRAM and kept both AST_MATCH and AST_UNOBSERVED in its break-only group, so a function defined inside an unobserved: block or a match arm was never reached, its body was never handed to check_unreachable, and the W003 emission never fired.

Both arms were missing here, so this adds both — unlike #790, where the walker already recursed AST_UNOBSERVED and only AST_MATCH was needed. The recursion idiom is check_builtin_shadow's from #787 verbatim: AST_MATCH descends case_count × body_counts[i] over bodies[i][j], AST_UNOBSERVED descends block.count over block.stmts[i].

Changes

• src/lint.c — two recursion arms added to check_func_unreachable immediately after the existing AST_PROGRAM arm; both labels removed from the break-only group. No new traversal helper, no change to the emission site, no other walker touched.
• tests/test_lint.sh — two tests, placed mid-file next to the existing W003 coverage rather than appended at the tail, since lint: W002 (unused parameter) never fires for a function defined inside unobserved: or a match arm #781 and lint: W010 (duplicate dict key) never fires inside unobserved: or a match arm #783 are adding tests to this same file.

Testing

• bash tests/test_lint.sh run directly for its exit code: 122 passed, 0 failed, 122 total, exit 0.
• Full suite tests/run_all_tests.sh: RESULTS: 3314/3314 passed, 0 failed.
• Both reproducers from the issue go from silent to warning; the control still warns exactly once.
• The tests bite. With src/lint.c reverted to upstream and the new tests kept, both fail (120 passed, 2 failed).
• Neither arm is dead. Neutralising AST_MATCH alone silences the match reproducer and fails exactly the match test; neutralising AST_UNOBSERVED alone silences the unobserved: reproducer and fails exactly that test.
• Verified against the merge result, not just the branch: rebased onto current main (1750b48), builds clean, tests/test_lint.sh exit 0.
• No new warnings across lib/ + examples/ + tests/ (397 .eigs files, each linted individually): zero W003 hits. fuzz/corpus/ was deliberately left out of that scope.

One note that will show in the logs but is not from this PR: src/lint.c already emits three compiler warnings on main (:825, :844, :911), all outside this diff.

Checklist

• make test passes locally — full suite via tests/run_all_tests.sh: 3314/3314 passed, 0 failed; lint tests run directly for their exit code, 122 passed, 0 failed, exit 0
• New builtins have signature comments and docs in docs/BUILTINS.md — n/a, no new builtins
• New library functions follow conventions in docs/STDLIB.md — n/a, no new library functions
• New examples have a comment header explaining what they demonstrate — n/a, no new examples
• CHANGELOG.md updated (if user-facing change) — not done, following the two merged precedents for this fix family: lint: recurse W012/W013 into unobserved blocks and match arms #787 and lint: recurse W001 collect_assigns into match arms #790 each changed only src/lint.c and tests/test_lint.sh. Happy to add an entry if you'd rather these carried one.

Generated by Claude Opus 5 (brief, review), Kimi K3 (implementation, verification)