refactor(sl): split overlay columns from column_overrides and enforce TS/Python wire contract

Overlay sources now have two distinct collections: `columns:` for computed
columns (requiring `expr` + `type`) and `column_overrides:` for metadata
patches to inherited manifest columns. Composing or loading an overlay that
mixes the two — or references an unknown column — fails with a typed error.

Introduce `ResolvedSemanticLayerSource` / `resolvedSourceSchema` /
`toResolvedWire` as the strict shape sent to the Python engine, and add a
schema contract test that diffs Zod against the Pydantic JSON schema dumped
by `python -m semantic_layer dump-schema`. `SourceDefinition` is now
`extra="forbid"` on the Python side.

`loadAllSources` surfaces per-file load errors instead of swallowing them,
so validation/query paths can report manifest shard parse failures.

pnpm-lock.yaml

@@ -376,6 +376,9 @@ importers:
       '@vitest/coverage-v8':
         specifier: ^4.1.6
         version: 4.1.6(vitest@4.1.6)
+      ajv:
+        specifier: 8.20.0
+        version: 8.20.0
       typescript:
         specifier: ^6.0.3
         version: 6.0.3