Fix resumable MCP backfill fold progress

[Widthdom]

Summary

• Make folded-key row rewrites durable before the final FoldReady stamp so interrupted rewrite-all backfills can resume.
• Add checkpoint metadata for rewrite-all symbol/reference phases and expose MCP backfill_fold progress.
• Update docs and add changelog fragment for MCP backfill_fold cannot resume after interrupt; no progress field #1461.

Validation

• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Debug -p:UseSharedCompilation=false -p:EnableAotAnalyzer=false -p:EnableTrimAnalyzer=false --filter "FullyQualifiedNameBackfillFoldedColumns|FullyQualifiedNameToolsCall_BackfillFold|FullyQualifiedName~RunBackfillFold_Cancelled"
• dotnet build src/CodeIndex/CodeIndex.csproj -c Debug -p:UseSharedCompilation=false -p:EnableAotAnalyzer=false -p:EnableTrimAnalyzer=false
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• codex adversarial review: No blocking/actionable issues found.

Note: running the same focused dotnet test without disabling AOT/trim analyzers fails on existing IL3050/IL2026 analyzer errors in unrelated JSON/MCP/config paths.

Fixes #1461