[codex] Expose bound model metadata

[TheSentinel454]

What changed

• Add DB.BoundModels() to expose read-only snapshots of bound model metadata for a datasource.
• Add value-only metadata structs for bound models, mapped columns, and keys/indexes.
• Include copied mapped column details such as AutoIncr, Nullable, and SQLType.
• Add unit coverage for metadata contents and slice-copy behavior.

Validation

• go test -vet=off ./... -count=1

Note: plain go test ./... -run TestDBBoundModelsReturnsReadOnlyMetadata -count=1 is blocked by an existing vet issue in builder_test.go: ExampleTable_Join refers to unknown field or method: Table.Join.