Fix fold readiness status gaps

[Widthdom]

Summary

• Add opt-in row-level fold readiness verification so a ready bit with incomplete folded rows degrades with fold_ready_bit_set_but_rows_incomplete.
• Cover every partial folded-column incomplete state in AllFoldedColumnsBackfilled.
• Mirror CLI fold remediation context in MCP status output when fold readiness is degraded.

Validation

• dotnet build CodeIndex.sln -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter "FullyQualifiedName~DbReaderTests|FullyQualifiedName~McpServerTests.ToolsCall_Status"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter "FullyQualifiedName~DbReaderTests.GetStatus_WithFoldRowVerification"
• dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false was started but did not complete in a bounded local run; it was stopped after prolonged no-progress output.
• Required adversarial review: codex exec --full-auto ... reported no blocking/actionable issues.

Documentation and Changelog

• DEVELOPER_GUIDE.md documents the new fold degradation reason and MCP/CLI fold remediation parity.
• Changelog fragments:
  • changelog.d/unreleased/1964.fixed.md
  • changelog.d/unreleased/1993.internal.md
  • changelog.d/unreleased/1973.fixed.md

Follow-up Candidates

• Local cdidx full index refresh was attempted twice and returned E012_INTERRUPTED; a partial touched-file refresh completed, but the local ignored .cdidx database remained degraded.

Fixes #1964
Fixes #1993
Fixes #1973