DuckDB bugbash: external source reads from files via `external_location`

## Goal
Compare dbt Core `dbt-duckdb` vs dbt Fusion DuckDB behavior for file-backed sources using `external_location`.

## Experiment
Start with local Parquet and CSV sources referenced through `external_location`, then validate source reads and downstream model behavior.

## Why this matters
`external_location` is one of the most visible and commonly discussed `dbt-duckdb` features in docs, tutorials, and blogs.

## Success criteria
- Equivalent source definitions work in both engines
- Any Fusion-specific limitation is captured in a minimal repro

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