feat(table): add partitions metadata table

[fallintoplace]

Summary

Adds the current-snapshot partitions metadata table to Table.Inspect.

• Aggregates data-file, position-delete, and equality-delete counts by partition.
• Preserves the latest update timestamp, snapshot ID, and partition specification ID.
• Coerces compatible evolved partition specs into a table-wide partition value.

Tests

• go test ./table
• Covers the partition schema, unified partition fields, and evolved partition specs.

Scope

This reports partition-level metadata for the current snapshot. It does not change file writing, delete application, or scan behavior.

Related

Builds on the shared content-file and partition representation in #1698.