fix: surface corrupt case metadata

[rogerchappel]

Summary

• distinguish a genuinely missing case index from malformed or unreadable metadata
• make list fail with an actionable diagnostic instead of silently dropping corrupt cases
• preserve the existing exit-2 behavior for missing cases and document plain-file recovery

Commits

• 74653cb test: cover corrupt case metadata
• c4e6be2 fix: surface unreadable case indexes
• 07e078c docs: explain index recovery errors

Verification

• npm ci
• npm run release:check (23 tests, typecheck, build, CLI smoke, package smoke, release contract)
• bash scripts/validate.sh
• git diff --check origin/main...HEAD

All commits are authored and committed by Roger Chappel miscanalysis@gmail.com.

[rogerchappel]

Automated merge note

Triage class: auto-merge

Summary: Surface malformed or unreadable plain-file case metadata, preserve missing-case behavior, and document recovery.

Checks run: CI test SUCCESS; local npm ci, npm run release:check, bash scripts/validate.sh, and git diff --check passed.

Rebased/CI-repaired: no.

Verified head SHA: 07e078ca23f25cac4e04b327f93402377c9dc931.