test(fix): add composing-fixes and collision edge-case tests

[protosphinx]

Why

The per-file buffer in applyFixes lets multiple rule fixes compose on
the same file in a single call (the second fix operates on the output of
the first, not the original raw content). This core feature had zero test
coverage: every existing test applies exactly one fix rule per file.

The jaccard collision detector also has a special-case branch
(if (a.size === 0 && b.size === 0) return 0) to guard against
divide-by-zero when both descriptions tokenize to empty sets (all words
are 2 chars or fewer). This path was also untested.

What

• test/fix-compose.test.ts (new file, 6 tests)
  • Compose name-drift and deprecated-tools-field on the same file
  • Compose tool-fields-ambiguous and tools-duplicate on the same file
  • Compose two rules across two files in one call
  • Verify notes array has one entry per applied fix when composing
  • Verify a diagnostic whose file is not in the parsed list is counted as skipped
  • Dry-run with composed fixes does not write to disk
• test/checks.test.ts (2 additions)
  • description-collision does not fire when both descriptions tokenize to empty sets
  • description-collision message includes the computed Jaccard score

Tests

• All existing 210 tests continue to pass
• 8 new tests added; total is 218

Self-merge gate

• all CI checks pass
• LOC delta < 250 (added + removed): 171 lines added, 0 removed
• no public-API surface change
• no runtime-dependency additions
• no workflow file changes
• tests added or extended: yes, new test file + 2 additions to checks.test.ts

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