Preserve status explain payloads in compact JSON

[Widthdom]

Summary

• Preserve the typed status --explain result in compact and byte-bounded JSON with a required five-field projection.
• Report the compact explanation schema and omitted optional fields in envelope metadata, while returning explicit E028_RESPONSE_BUDGET_TOO_SMALL recovery guidance when one row cannot fit.
• Cover static, generated, dotted dynamic, and label-alias explanations together with text, unknown-field, explicit projection, and compact-format behavior.

Validation

• dotnet restore CodeIndex.sln
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false (net8.0: 11,499 passed / 7 skipped; net9.0: 11,023 passed / 422 skipped)
• dotnet format CodeIndex.sln --verify-no-changes --no-restore --verbosity diagnostic
• Focused status/projection tests, including the final adversarial review run (81 passed on each TFM)
• dotnet run --project tools/CodeIndex.Changelog -- check
• Codex adversarial review: no blocking/actionable issues found after synchronizing the authoritative status contract.

Documentation and changelog

• Updated README.md, DEVELOPER_GUIDE.md, TESTING_GUIDE.md, and AGENT_GUIDE.md.
• Added bilingual fragment changelog.d/unreleased/5093.fixed.md.

Follow-up candidates

None.

Fixes #5093