Two correctness/validation improvements to managed materialization:
1. A keyed `merge` (`key=<col>`) is delete-by-key + insert-all and does NOT
deduplicate its source, so two incoming rows sharing a key both landed
under that key — silently breaking the one-row-per-key contract. Codegen
now emits an in-transaction guard (same `error(...)` shape as the schema
-drift guard) that fails the run when the SELECT returns more than one row
for a non-NULL key, naming the key. Authors deduplicate in the SELECT or
switch to `append`. NULL keys are exempt, matching the delete's `IN (...)`
scope.
2. The two SCD2 misconfigurations that were only caught at run time — `history`
without `key=`, and `history` + `// partitioned` — now fail fast at deploy
via a shared `MaterializeSpec::validate`, called from `create_script_internal`.
The DuckDB executor keeps the same check as a safety net for preview/test
runs that never deploy (shared message, no drift).
Adds unit tests for the merge guard codegen and for `validate` (all four
cases), and updates docs/ducklake-materialization.md and docs/pipelines-vs-dbt.md.
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Ruben FiszelandClaude Opus 4.8
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