Reject invalid index numeric options

[Widthdom]

Summary

• Reject invalid explicit numeric index options with structured E010_USAGE_ERROR output before database setup or worker startup.
• Keep compatible environment fallback/clamping for indexing paths while reporting the environment source and effective value in human and JSON warnings.
• Suppress those fallback warnings in --optimize mode, where the numeric indexing settings are not consumed.
• Document the inclusive ranges and fallback policy, and add boundary, provenance, duplicate, no-mutation, output-contract, and watch JSON coverage.

Validation

• dotnet restore CodeIndex.sln
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-restore (0 warnings, 0 errors)
• Full Release tests: .NET 8 — 9,013 passed, 6 skipped; .NET 9 isolated rerun — 11,239 passed, 423 skipped, 0 failed after the concurrent test host process exited unexpectedly
• Issue-focused tests after the final review fix: .NET 8/9 — 77 passed per target, 0 failed
• Ambient-environment isolation and watch JSON contract tests: .NET 8/9 — 3 passed per target, 0 failed
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet run --project tools/CodeIndex.Changelog -- check
• git diff --check
• Root index check and workspace status: healthy, 2/2 members ready
• Codex adversarial review: two review loops completed; all four P2 findings were fixed and the final optimize JSON reproduction was rechecked without an unstructured warning

Documentation and changelog

• Updated the English and Japanese indexing contracts in DEVELOPER_GUIDE.md and TESTING_GUIDE.md.
• Updated generated CLI help range/fallback descriptions.
• Added bilingual fragment changelog.d/unreleased/5097.fixed.md.

Fixes #5097

Follow-up candidates

• None.