No versioning policy for codeindex_meta key additions; silent stale-key accumulation

[Widthdom]

Summary

The codeindex_meta table stores versioning metadata for multiple independent subsystems: fold_key_version/fingerprint (#86), CSharpSymbolNameContractVersion (#435), SqlGraphContractVersion, HotspotFamilyVersion (per-language), and MetadataTargetVersion (per-language). There is no mechanism to detect or clean up stale keys when a new binary version deprecates or renames a key. ClearHotspotFamilyReady() at line 887 manually sets specific keys to null, but no systematic policy exists. A future version bump that renames fold_key_version to fold_schema_version will leave the old key behind indefinitely, creating confusion and silent feature-detection bugs if readers check only the new key.

Where

• src/CodeIndex/Database/DbWriter.cs:887-898 (ClearHotspotFamilyReady sets keys to null manually)
• src/CodeIndex/Database/DbWriter.cs:869-879 (MarkHotspotFamilyReady cleanup example)
• src/CodeIndex/Database/DbContext.cs:672-680 (codeindex_meta table definition)

Suggested approach

1. Add a codeindex_meta_schema_version key that increments when the set of recognized keys changes
2. Build a static registry of "expected keys" keyed by binary version number
3. When opening the DB, compare the stored codeindex_meta_schema_version with the binary's expected version
4. If they differ, apply a migration that removes unrecognized keys and adds missing ones with default values
5. Document this policy in a SCHEMA.md file alongside versioning rules
6. Add a cdidx status --detailed subcommand that dumps all codeindex_meta contents for debugging

[Widthdom]

Work started on branch fix-issue2025-2026-1988-2037.