Classify scoped JSON trust boundaries

[Widthdom]

Summary

• classify JSON read/write recipe findings from explicit origin, direction, sensitivity, trust, and rationale annotations
• mark the private local JSONL unsafe-relaxed writer with a controlled trust rationale while keeping external and untrusted findings visible
• lexically bind annotations to retained C# operations across selected JSON child queries, overlapping API substrings, verbatim identifiers, composite casts, conditional assignments, expression-bodied default parameters, multiline continuations, qualified declarations, generic wrappers, and conditional-compilation boundaries
• keep source reconstruction bounded and skip it for output shapes that do not serialize classifications

Validation

• make lint
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet build CodeIndex.sln -c Release --no-restore -p:UseSharedCompilation=false
• .NET 8 full suite: 11,096 passed and 7 skipped; three unrelated load/timing failures during simultaneous CodeIndex3rd net8/net9 full-suite load all passed on exact isolated retry
• .NET 9 full suite: 10,624 passed and 420 skipped; one unrelated parallel-batch timeout during the same simultaneous load passed on exact isolated retry
• Issue Let unsafe-relaxed JSON audits express scoped private trust boundaries #4913 focused tests: 3/3 passed on both .NET 8 and .NET 9 after the final adversarial-review fixes, in Debug and Release
• CodeIndex index: 1,347 files, 46,905 symbols, and 465,193 references; exact HEAD freshness and all readiness checks passed
• codex review --base origin/main: no blocking or actionable defects identified; the solution builds cleanly and the targeted tests pass on net8.0 and net9.0

Documentation and changelog

• Updated the English and Japanese sections of USER_GUIDE.md, DEVELOPER_GUIDE.md, and TESTING_GUIDE.md.
• Added bilingual changelog.d/unreleased/4913.security.md.

Follow-up candidates

None.

Fixes #4913