Fix impact count totals independent of limits

[Widthdom]

Summary

• Make impact count modes independent of presentation limits and avoid materializing normal result rows.
• Batch bounded identity-target traversal so multi-hop MCP counts complete within the request deadline while preserving graph-readiness and lower-bound authority signals.
• Report MCP presentation limits as ignored in count-only mode, and add bilingual documentation, changelog, and regression coverage.

Testing

• dotnet build CodeIndex.sln --no-restore -p:UseSharedCompilation=false (Debug; 0 warnings, 0 errors)
• Full Release .NET 8 suite: 12,114 passed, 9 skipped; one unrelated allocation microbenchmark failed once and passed on an immediate isolated rerun (1/1)
• Full Release .NET 9 suite: 11,630 passed, 431 skipped, 0 failed
• Impact test suite: 141 passed on each of .NET 8 and .NET 9
• dotnet format CodeIndex.sln --no-restore --verify-no-changes
• Changelog fragment validation and git diff origin/main...HEAD --check
• Codex adversarial review: 2 rounds (workflow maximum); all findings from both rounds were fixed
• Representative 300-result CLI count reduced from about 54 seconds to 8.38 seconds; the equivalent MCP request completed without timing out

Fixes #5226