DuckDB bugbash: external materialization and upstream registration flow

## Goal
Compare dbt Core `dbt-duckdb` vs dbt Fusion DuckDB behavior for external materialization workflows.

## Experiment
Cover:
- writing Parquet output via external materialization
- a partitioned output case
- re-reading upstream external models, including `register_upstream_external_models()`-style flow where applicable

## Why this matters
External materialization is one of the adapter's distinctive workflow features.

## Success criteria
- Equivalent external model workflows operate in both engines, or
- a minimal repro isolates the Fusion gap

## Notes
Exclude static-analysis / `SQL_understanding` concerns from scope.