Use structural evidence for safety audit guards

[Widthdom]

Summary

• replace line-proximity safety-audit guards with container-scoped structural evidence
• require same-path whole-file size/control relationships and resolved awaited bounded-writer helpers
• resolve the EnumerationOptions value actually passed to Directory.Enumerate*, including named arguments, lexical scope, and shadowing
• expose accepted and rejected guard decision, reason, relationship, subject, container, and evidence-path metadata
• add regression coverage for unsafe controls, named arguments, reassignment, conditional sources, switch dominance, helper binding, and option shadowing

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Debug -f net8.0 --no-restore --filter FullyQualifiedName~Issue4912 (3 passed)
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Debug -f net9.0 --no-restore --filter FullyQualifiedName~Issue4912 (3 passed)
• full net9.0 suite (10,574 passed, 420 skipped, 0 failed)
• full net8.0 suite (11,047 passed, 7 skipped; one performance smoke exceeded its 20-second budget by 0.3 seconds under concurrent net9.0 load), followed by an isolated rerun of that performance smoke (passed in 3 seconds)
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet run --project tools/CodeIndex.Changelog -- check (86 fragments validated)
• dotnet build CodeIndex.sln -c Debug --no-restore (0 warnings, 0 errors)
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll audit risky-code/file-read-all-text --allow-partial --format compact (0 findings)
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll audit phrase-risk-patterns/readalltext-call-site --allow-partial --format compact (0 findings)
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll audit filesystem-traversal/enumerate-without-options --allow-partial --format compact (4 remaining reviewable findings; 3 structurally safe calls excluded)
• post-commit index . --commits HEAD --json (1,324 files, 11,070 chunks, 46,618 symbols, 458,577 references; 0 warnings/errors; index and reference graph complete/current)
• two codex review --base origin/main adversarial-review rounds completed; all 15 actionable findings were fixed and regression-tested

Documentation and changelog

• updated the English and Japanese sections in USER_GUIDE.md and TESTING_GUIDE.md
• added changelog.d/unreleased/4912.fixed.md

Follow-ups

• None.

Fixes #4912