feat: add refs metadata table

[gmhelmold]

Which issue does this PR close?

• Part of [EPIC] feat: support metadata tables #823.

First table slice of the metadata-tables epic, as announced on the epic thread (refs / history / metadata_log_entries are the opening slices).

What changes are included in this PR?

Adds the refs metadata table, exposing a table's snapshot references (branches and tags) as rows, matching Iceberg's reference RefsTable.java.

Schema (field ids 1–6, identical to the reference SNAPSHOT_REF_SCHEMA):

id	column	type	required
1	name	string	✓
2	type	string	✓
3	snapshot_id	long	✓
4	max_reference_age_in_ms	long	optional
5	min_snapshots_to_keep	int	optional
6	max_snapshot_age_in_ms	long	optional

• type is the uppercase ref kind — "BRANCH" / "TAG" — matching Java's SnapshotRefType.name().
• Retention columns follow the reference mapping: a branch fills min_snapshots_to_keep, max_snapshot_age_in_ms, and max_reference_age_in_ms; a tag fills only max_reference_age_in_ms, leaving the other two null.
• Rows are emitted name-ordered. TableMetadata::refs is a HashMap, so the scan collects into a BTreeMap to keep output deterministic.
• No public getter is added; the scan reads the existing pub(crate) field directly, so the public surface is unchanged.

Registry wiring: MetadataTableType::Refs (+ as_str / TryFrom), the MetadataTable::refs() accessor, and the DataFusion TableProvider schema/scan arms. The $refs DataFusion table name is enumerated automatically via MetadataTableType::all_types() (strum EnumIter).

Are these changes tested?

Yes.

• Two unit goldens in inspect::refs::tests:
  • test_refs_table uses the shared TableTestFixture (a main branch auto-injected by V2 normalization with all-null retention, plus a test tag). It pins row order, the BRANCH/TAG type strings, and the tag's max_reference_age_in_ms.
  • test_refs_table_retention_fields uses inline metadata (TempDir-based, deterministic) whose main branch carries explicit retention — the load-bearing golden for the branch column mapping (min_snapshots_to_keep=5, max_snapshot_age_in_ms=86400000, max_reference_age_in_ms=259200000), which the all-null fixture branch does not exercise.
• DataFusion / sqllogictest goldens: test_provider_list_table_names now lists my_table$refs; show_tables.slt lists the $refs tables; insert_into.slt selects $refs on a snapshot-less table and expects zero rows (V2 normalization only injects main once a current snapshot exists).

Red→green verified locally: flipping the branch type string "BRANCH" → "branch" fails both unit goldens on the type column; restoring passes. cargo clippy --all-targets --all-features --workspace -- -D warnings clean.

Are there any user-facing changes?

New feature. SELECT * FROM <table>$refs (via DataFusion) and table.inspect().refs() now expose a table's snapshot references.