Explain every serialized status section

[Widthdom]

Summary

• derive accepted status --explain keys from the source-generated StatusResult serializer registry
• provide structured meaning, source, dependencies, interpretation, and repair guidance for major and nested status sections
• bound and sanitize unknown-key candidates, register explain fields for --fields, and keep bounded explain envelopes free of runtime/path metadata
• preserve the shared FTS optimization explanation from the latest main

Root cause

The explain command used a hand-maintained allowlist that could drift from the serialized status response, and the generic bounded-response wrapper added runtime metadata to otherwise static explanations.

Validation

• dotnet build CodeIndex.sln -c Release --no-restore — 0 warnings, 0 errors
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• full Release net8 suite — 10,927 passed, 0 failed, 7 skipped
• full Release net9 suite — 10,453 passed, 0 failed, 420 skipped
• post-review focused net8/net9 suites — 94 passed per framework
• manual bounded projection/redaction reproduction
• changelog validator — 68 fragments validated
• local index/status check — workspace matched; index and graph ready
• Codex adversarial review — 2 rounds completed; all findings addressed

An additional review-only Debug full-suite run encountered a test-generated unreadable fixture in LocalJsonlJsonWriterOptionsTests; the canonical Release suites above were green, and the affected focused tests passed after the final correction.

Documentation and changelog

Updated the bilingual README, Developer Guide, Testing Guide, and AGENT_GUIDE.md. Added changelog.d/unreleased/4891.fixed.md.

Follow-up candidates

None.

Fixes #4891