Estimate index dry-run mutations

[Widthdom]

Summary

• Replaces misleading all-zero index --dry-run mutation counts with bounded parse-only estimates for changed/new parsable sources.
• Emits per-metric source, confidence, and explicit unknown reasons whenever a trustworthy value cannot be produced.
• Projects updates, skips, policy skips, deletes, stale-row purges, cap hits, and bounded-estimation truncation without mutating the database or source tree.

Implementation impact

• Reuses production invalidation contracts when deciding whether an indexed file can be projected as unchanged.
• Limits parse estimation to 100 update candidates and marks affected child metrics unknown when the limit, path cap, parse failure, partial schema, snapshot failure, or cross-file C# augmentation prevents a reliable estimate.
• Covers JSON and human output while preserving dry-run's non-mutating behavior.

Validation

• Release build: net8.0/net9.0, 0 warnings and 0 errors.
• Full net8.0 suite: 10,925 passed, 7 skipped, 0 failed (10,932 total).
• Full net9.0 suite: 10,451 passed, 420 skipped, 0 failed (10,871 total).
• Focused Issue Return useful or explicit-unknown mutation estimates from index dry-run #4893 tests: 11/11 passed on net8.0 and net9.0.
• Dry-run regression group: 44/44 passed on net8.0; 42 passed, 2 skipped, 0 failed on net9.0.
• dotnet format CodeIndex.sln --verify-no-changes --no-restore passed.
• Changelog fragment validation passed.
• git diff --check passed.

Adversarial review

Two required Codex adversarial-review rounds were completed. Round 1 findings around persisted caps, production reuse contracts, and unchanged binary projections were fixed. Round 2 findings around forced extractor invalidation, defensive symbols-only handling, C# workspace augmentation, hotspot marker trust, and unavailable snapshot reporting were fixed. (--symbols-only --dry-run is already rejected by command validation; the reuse helper also handles it defensively.)

Documentation

Updated the English and Japanese README/developer-guide sections and added changelog.d/unreleased/4893.changed.md.

Fixes #4893