fix(retain): sync metadata to unchanged memories

[Sanderhoff-alt]

Summary

Keep metadata on memory units aligned with document_metadata when delta retain preserves unchanged chunks.

Closes #3008.

Root cause

Delta retain already updated the document row and propagated tags to existing memory units. It did not propagate metadata, so preserved memories kept the previous value while rebuilt or newly inserted memories used the current retain metadata.

Chunk state	Existing behavior	Result after this change
unchanged	Preserve memory units with stale metadata	Preserve memory units and synchronize metadata and tags
changed	Delete and rebuild with current metadata	Unchanged
new	Insert with current metadata	Unchanged
removed	Delete chunk and memory units	Unchanged

Implementation

• Replace the tags-only delta helper with one focused update that synchronizes metadata and tags in a single SQL statement.
• Reuse that helper in metadata-only retain and partial-delta retain.
• Keep changed/new insertion and changed/removed deletion paths unchanged.
• Add regression coverage for metadata-only replace and append with preserved plus new chunks.

Impact

After retain completes, every surviving direct memory unit for the document has metadata consistent with document_metadata, regardless of whether delta retain preserved or rebuilt its chunk.

The change adds no schema migration, JSON-specific branch, or extra database round trip.

Validation

Check	Result
Delta retain and append test files	29 passed
Focused metadata and tags regressions after simplification	3 passed
Ruff	Passed
ty	Passed
Repository pre-commit hooks and lint	Passed
git diff --check	Passed