Explain and safely narrow C# incremental update expansion

[Widthdom]

Small C# scoped refreshes could re-extract nearly every C# file because static-interface and member-reference contracts required workspace expansion. This change explains that work and narrows re-extraction only when complete source-input evidence proves the workspace lookup inputs unchanged.

Fixes #5347

Changes

• Add bounded csharp_workspace_expansion diagnostics with the trigger, decision, original/expanded/final target counts, and preflight phase costs. Successful CLI updates and persisted CLI/MCP status share the same object; configuration-driven full scans identify their fallback.
• Preserve the complete workspace scan and source/configuration barriers. An optional, run-bound fingerprint permits a narrower main pass for independent instance-only edits. It binds project-marker evidence from the same validated expansion scan. Missing evidence, changed contributing sources/order, configuration or binary builds, hooks/custom extractors, filters, incomplete readiness, and races retain conservative behavior.
• Keep rename/deletion cleanup, untouched references, generated suppression, cancellation, and verified-HEAD provenance. Full/other indexing paths invalidate the optional proof; existing databases require no migration.

Validation

• CI formatting follow-up: make lint passed after formatting two C# files; the Release test-project build passed with zero warnings/errors, and the affected metadata regression passed on both .NET 8 and .NET 9. This follow-up changes whitespace only.
• Release solution build: zero warnings/errors.
• Relevant update/status/JSON snapshot regressions: .NET 8 208 passed; .NET 9 205 passed, 3 existing framework skips.
• Final focused run: 20 passed on each of .NET 8 and .NET 9, comprising thirteen new Explain and safely narrow C# workspace expansion during incremental refresh #5347 cases and seven JSON snapshots. Coverage includes full-index symbol/graph parity, Git verification, contracts/members, renames/removals, configuration/generated/filter transitions, cancellation/races, bounded metadata, fingerprint ordering/budgets, project-marker changes and budget exhaustion between scans, case-only rename peak counts, and quiet fallback output.
• Changelog validation and git diff --check passed.
• Isolated 99-C#-file Git fixture, using an old-ref database for each comparison: an independent edit updated 1 file / 1,777 bytes / 1,850 ms, versus 99 / 170,819 / 2,720 ms for the same edit with the optional proof removed. A shared member-target rename retained expansion: 99 / 170,821 / 3,478 ms. These are file-loop bytes and persisted durations; the C# prepass still examined about 171 KB. Source/graph data and freshness matched the conservative/full controls. Full measurements and limitations are in the documentation; these single runs do not establish a general speedup.
• Exact historical workload reproduced in a detached checkout with fresh binaries at both refs and a private database verified at the old ref: 24 Git paths → 1,424 scanned, 1,410 updated, 14 removed, 39,061,997 bytes, 154,465 ms persisted duration. Root/manifest freshness and source/graph completeness passed at both refs.

Documentation

• Bilingual contract and measurement notes: docs/csharp-update-expansion.md, with English/Japanese developer/testing-guide updates.
• Changelog fragment: changelog.d/unreleased/5347.changed.md; CHANGELOG.md is unchanged.
• AGENT_GUIDE.md updates the shared status contract. No agent entry-point files were changed.

Review and scope

Two Codex adversarial passes identified project-marker proof, case-only rename diagnostics, quiet-output behavior, and a follow-up marker-scan race. All findings were fixed with behavioral regressions. The final race/budget cases fail before the fix and pass afterward, with conservative/full extraction controls. Review followed the repository's two-round limit; the final fix was verified by tests. Final root/manifest freshness and completeness checks passed at the committed HEAD, and immediate/persisted expansion diagnostics matched. Only #5347 is in scope; no unrelated follow-up issue was identified.