Fix watch JSON summaries and dry-run mutation estimates

[Widthdom]

Summary

• Add structured cdidx index --watch --json batch metadata, bounded path samples, sub-run count parsing, parse diagnostics, and overflow recovery commands that preserve DB/indexing options.
• Add cdidx index --dry-run --json mutation estimates for projected updates/deletes/purges, unsupported/unknown counts, and table mutation counts without writing the DB.
• Align dry-run purge estimates with full-scan safety, partial rename cleanup, checksum rename cleanup, and Unicode-normalized DB paths.
• Add changelog fragments for Watch mode JSON should expose batch summaries, parse gaps, and overflow recovery #3568 and Index dry-run should estimate projected deletes and table mutations #3569.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~Run_DryRun_WithFiles_NormalizesUnicodeDbPathForEstimates|FullyQualifiedName~Run_DryRun_WithFiles_ReportsChecksumRenamePurgeWithoutWriting|FullyQualifiedName~Run_DryRun_WithFiles_ReportsSupportedExtensionRenamePurgeWithoutWriting|FullyQualifiedName~Run_DryRun_WithFiles_ReportsUnsupportedExtensionRenamePurgeWithoutWriting" -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~IndexWatchRunnerTests|FullyQualifiedName~Run_DryRun_WithFiles_ReportsProjectedUpdatesDeletesAndUnknowns|FullyQualifiedName~Run_DryRun_WithFiles_NormalizesUnicodeDbPathForEstimates|FullyQualifiedName~Run_DryRun_WithFiles_ReportsChecksumRenamePurgeWithoutWriting|FullyQualifiedName~Run_DryRun_WithFiles_ReportsSupportedExtensionRenamePurgeWithoutWriting|FullyQualifiedName~Run_DryRun_WithFiles_ReportsUnsupportedExtensionRenamePurgeWithoutWriting|FullyQualifiedName~Run_DryRun_FullScan_ReportsProjectedPurgesWithoutWriting|FullyQualifiedName~Run_DryRun_FullScan_DoesNotProjectUnreadableSubtreePurge|FullyQualifiedName~Run_DryRun_FullScan_ReportsUnreadableDirectory" -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• git diff --check
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll index . --changed-between origin/main HEAD --json
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• codex exec --full-auto ... adversarial review: No blocking/actionable issues found.

Fixes #3568
Fixes #3569