Stabilize startup handoff watch test

[Widthdom]

Summary

• verify startup-handoff reconciliation at the watch readiness boundary
• accept the equivalent transcript where the baseline rescan already observed the queued mutation
• document the bilingual fixture contract and add the changelog fragment

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --no-restore --filter "FullyQualifiedName~RunCore_StartupHandoff_ReconcilesMutationAndDrainsBeforeReady_Issue4594" -p:UseSharedCompilation=false (net8.0 / net9.0)
• three concurrent --no-build multi-target targeted runs (6/6 passed)
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet build CodeIndex.sln -c Release --no-restore -p:UseSharedCompilation=false
• dotnet test --no-restore -p:UseSharedCompilation=false (net8.0: 11,085 passed, 7 skipped; net9.0: 10,611 passed, 420 skipped)
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Codex adversarial review: no blocking or actionable findings

Documentation and changelog

• updated the English and Japanese sections of TESTING_GUIDE.md
• added changelog.d/unreleased/4939.fixed.md

Follow-up candidates

None.

Fixes #4939