Expose completion metadata in ad-hoc SARIF

[Widthdom]

Summary

• expose source, returned, omitted, truncation, limit, cursor-availability, and replay metadata on ad-hoc search SARIF runs
• count the final SARIF result/location unit, including exact-substring occurrence expansion
• keep guarded recounts bounded and label their observed source counts non-authoritative
• preserve raw FTS, option-like queries, filters, guards, and normalized search controls in replay commands
• add complete, limited (1 of 126), empty, occurrence-expanded, guarded, and merged-run regression coverage
• document the contract in both English and Japanese and add a changelog fragment

Root cause

The ad-hoc search path called the shared SARIF writer without run properties and applied result limits after occurrence expansion without exposing completion accounting. As a result, consumers could not distinguish complete, empty, and truncated SARIF runs or safely reproduce the query.

Validation

• dotnet restore CodeIndex.sln --locked-mode
• dotnet build CodeIndex.sln -c Release --no-restore — 0 warnings, 0 errors
• focused search/SARIF/selector tests — net8.0: 11 passed; net9.0: 11 passed
• full Release suite — net8.0: 10,608 passed, 7 skipped, 0 failed; net9.0: 10,135 passed, 419 skipped, 0 failed
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• changelog validation — 22 fragments validated
• final workspace index check — 1,232/1,232 files matched, index_matches_workspace=true
• Codex adversarial review round 1: 3 findings fixed (occurrence counts, raw-FTS replay flag, option-like query replay)
• Codex adversarial review round 2: 2 findings fixed (bounded guarded recounts, exhaustive facet-filtered exact counts)

A parallel full-suite run during development exposed an unrelated 5-second Unicode worker timeout; the isolated rerun and the final full suite passed. Related: #4937

Documentation and changelog

• updated DEVELOPER_GUIDE.md and TESTING_GUIDE.md in both English and Japanese
• added changelog.d/unreleased/4844.fixed.md
• did not modify CHANGELOG.md, AGENTS.md, CLAUDE.md, or AGENT_GUIDE.md

Follow-up candidates

• Stabilize net9 UTF-8 symbol-worker test under parallel full-suite load #4937

Fixes #4844