Unify catalog generation and typed data identity

[zzylol]

Why

The SDS contract represented one catalog snapshot with both CatalogGeneration and SummaryCatalogReference, using different digest field names. DataDescriptor also identified every population by a metric-name string, so a time-series metric and a relational table could collide and SQL value-column changes did not change descriptor identity.

Before this PR

Catalog consumers converted between two equivalent generation structs. DataDescriptor(metric_name, filter, group_by, observation_semantics) lost whether the source was a metric or table and which value was summarized.

After this PR

CatalogGeneration is the single immutable snapshot identity used by the catalog, plans, inventories, and instances. Its canonical digest field is snapshot_sha256; the old snapshot_digest spelling is accepted only as a deserialization alias.

DataDescriptor now carries language-neutral typed identities:

• DataSourceIdentity::TimeSeries { metric } or Table { table_ref }
• ValueProjectionIdentity::SampleValue or Column { name }

Both are included in the content-addressed descriptor ID. The catalog schema advances to version 2 because its canonical descriptor shape and identity changed. SummaryStore indexes only time-series descriptors by metric while retaining relational descriptors without forcing them into that index.

Example

A metric named events, table events.value, and table events.cost now produce three distinct DataDescriptorId values. They can share an operator descriptor while remaining distinct summarized populations.

Verification

• cargo check --workspace --all-targets
• cargo test -p asap_types sds::tests (14 passed)
• cargo test -p control_plane --lib physical::compiler::tests (67 passed)
• cargo test -p control_plane --lib summary_reconcile (3 passed)
• cargo test -p data_plane --lib storage_engines::sketch_db::sds (5 passed)
• git diff --check

Depends on #572.

ClickHouse migration surface: use DataSourceIdentity::Table with an explicit ValueProjectionIdentity::Column; publish and persist the same CatalogGeneration returned by SummaryCatalog::reference().