docs(formal): index the challenges and mutants; read an entry with where.mjs

[lan17]

Phase 3 of #220, item D1, reshaped after review. One commit, stacked on #233 until it lands, then retargeted to main.

What changed since the first version

The first version mirrored the challenge and mutant catalogs into 8,190 lines of Markdown. That earned its lines badly: a 345 KB mirror is no easier to read than the JSON it copies, it costs a regeneration on every catalog edit, and it is worse for an agent, which parses JSON fine. What a reader needs is a skimmable list and one entry at a time.

What this does now

• formal/generated/CHALLENGES.md (91 lines) and MUTANTS.md (76 lines): one row per challenge (contract, source, invariant, native mutant, reproducer run, detected and excluded counts) and per mutant (case, description, detections per port, challenges mapped to it). Generated by the same --write, checked by the same --check in the audit lane.
• where.mjs reads an entry in full on demand: node formal/tools/where.mjs recovery-inclusive-maximum prints the edit, the invariant, the mapping with its cross-contract note and evidence checkpoint, the reproducer and every exclusion with its reason; node formal/tools/where.mjs M01 prints the rationale, the edits per port including the Rust mirror, the required detections and the challenges mapped to it.

Net: +167 generated lines instead of +8,190, no new reading surface, and the lookup the mirror was really for.

Tests

formal-catalog-docs.test.ts pins all three views to the renderers and checks that every challenge and every mutant has one row and that the indexes stay within a few lines of the entry count. formal-where.test.ts covers the challenge and mutant lookups and their error paths.

Verification

make audit green; vitest run test/formal-catalog-docs test/formal-where pass; tsc --noEmit clean; node scripts/check-docs.mjs green (32 pages). Indexes and AUTHORING re-keyed with --refresh; parity inputs re-pinned.

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.73%. Comparing base (e75a04b) to head (a611a51).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #235   +/-   ##
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  Coverage   93.73%   93.73%           
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  Files          78       78           
  Lines       11357    11357           
  Branches      656      656           
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  Hits        10645    10645           
  Misses        642      642           
  Partials       70       70           

Flag	Coverage Δ
go	91.12% <ø> (ø)
python	92.08% <ø> (ø)
rust	94.29% <ø> (ø)
typescript	96.56% <ø> (ø)

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