[Subtask] Complete V3 type compatibility for the Paimon catalog

### Describe the subtask

Complete and document the five-family compatibility contract for Paimon 1.2.

**Compatibility outcomes**

- **Nanosecond timestamps:** Round-trip exactly through Paimon precision-9 timestamp types; reject precision 10 through 12.
- **Variant:** Round-trip through Paimon's native Variant type.
- **Unknown/Null:** Reject.
- **Geometry:** Reject.
- **Geography:** Reject.

**Acceptance**

- Reproduce all five families through the connector's public create/conversion path and its alter/load paths where supported.
- A supported mapping must round-trip without losing type identity or parameters.
- An unsupported mapping must return `IllegalArgumentException` before external metadata mutation; no fallback to a narrower or opaque type is allowed.
- Cover nested types where the connector supports complex columns.
- Add focused converter/operation tests and an integration or no-side-effect test appropriate to the connector.
- Update the connector documentation with a five-family compatibility matrix and rejection rationale.
- Keep implementation commits atomic by connector and type family.

**Connector-specific verification**

Reproduce supported cases with a real filesystem catalog create/load round trip. Verify rejected create and mixed alter operations leave columns and properties unchanged, then run the focused and full module suites.

**Documentation**

`docs/lakehouse-paimon-catalog.md`

### Parent issue

#12056