Make database checkpoint plans fully predictive

[Widthdom]

Summary

• introduce an immutable checkpoint plan shared by dry-run output and execution, covering DB/WAL/SHM inputs, every planned payload, the versioned manifest, hashes, byte estimates, conflicts, policies, and uncertainty
• verify source fingerprints before and after copying, verify copied artifacts, reject drift before atomic publication, and reject non-regular inputs such as Unix FIFOs before hashing
• preserve legacy JSON counters while adding detailed plan fields, expand human output and command help, and document the contract in both English and Japanese
• add the required changelog fragment and focused regression coverage for parity, drift, conflicts, manifest collisions, missing sidecars, and FIFO inputs

Validation

• dotnet build CodeIndex.sln -c Release --no-restore (0 warnings, 0 errors)
• dotnet format CodeIndex.sln --no-restore --verify-no-changes
• dotnet run --project tools/CodeIndex.Changelog/CodeIndex.Changelog.csproj -c Release --no-restore -- check (68 fragments)
• DbCommandRunnerTests: net8 95/95; net9 95/95
• issue-focused tests: net8 8/8; net9 8/8
• full net8 suite: 10,922 passed, 7 skipped, 0 failed
• full net9 suite: 10,448 passed, 420 skipped, 0 failed
• Codex adversarial review: 2 rounds completed; all findings addressed

Fixes #4890