Fix durable suggestion writes

[Widthdom]

Summary

• Flush suggestion temp-file writes to disk before replacing the live suggestions file.
• Preserve zero-byte suggestion stores as .bak files instead of treating them as empty.
• Add regression coverage and an unreleased changelog fragment for SuggestionStore.SaveUnlocked has no fsync; power loss can silently wipe local suggestions #1410.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter SuggestionStoreTests
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --filter SuggestionStoreTests -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• codex adversarial review: No blocking/actionable issues found.

Note: dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false was attempted before rebasing onto the updated origin/main, but it produced no completion output for over 8 minutes and was stopped; focused Release tests passed afterward.

Documentation and changelog

• Added changelog fragment: changelog.d/unreleased/1410.fixed.md
• No documentation updates were needed; this preserves the existing suggestion-store interface and hardens persistence/recovery behavior.

Follow-up candidates

• None.

Fixes #1410