Fix qualified C# graph identity resolution

[Widthdom]

Summary

• Resolve exact qualified C# caller, caller-count, and impact queries against definition identities while preserving verified polymorphic dispatch.
• Count current C# callable hotspots by resolved logical target identity through a bounded identity-aware candidate frontier.
• Keep caller path filters scoped to returned callers, constrain common-call opt-in fallback, and avoid authoritative zero results when identity expansion reaches a safety budget.
• Add regression coverage for qualified-name collisions, unresolved names, path scoping, bounded hotspots, and ambiguity handling.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-restore (0 warnings, 0 errors)
• Targeted graph/hotspot regression set on .NET 8 and .NET 9 (100/100 passed on each target)
• Full Release suite:
  • .NET 9: 10,968 passed, 418 skipped, 0 failed
  • .NET 8: 11,439 passed, 7 skipped, 1 unrelated timing-sensitive BoundedRegexTests failure; the failed test passed immediately when rerun alone on both .NET 8 and .NET 9
• dotnet run --project tools/CodeIndex.Changelog -- check
• git diff --check
• Root and workspace dogfood index checks after indexing HEAD and completing fold backfill

Review

• Completed the required two-round Codex adversarial review and addressed every actionable finding from both rounds.

Documentation and changelog

• Updated the English and Japanese bounded-hotspot documentation in DEVELOPER_GUIDE.md.
• Added changelog.d/unreleased/5084.fixed.md.

Follow-up candidates

• BoundedRegex consumer-break test can flake during the full parallel suite #5113

Fixes #5084