Fix DB compatibility docs and checkpoint rollback

[Widthdom]

Summary

• Add bilingual DB compatibility policy documentation and link it from README/DEVELOPER_GUIDE.
• Add db checkpoint/list/restore commands and automatic backfill-fold checkpointing.
• Harden restore against incomplete checkpoints and restore failures.

Issues

Fixes #1718
Fixes #1721

Follow-up: #2747

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter DbCommandRunnerTests
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter DbCommandRunnerTests --no-restore
• dotnet run --project tools/CodeIndex.Changelog -- check
• git diff --check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Codex adversarial review: No blocking/actionable issues found.

Full solution test run was attempted earlier but encountered unrelated intermittent failures/stall; tracked separately in #2747.