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.

packages/context/src/sl/tools/sl-warehouse-validation.test.ts

@@ -8,7 +8,7 @@ function makeDeps(opts: { sourceYaml: string; executeQuery: ReturnType<typeof vi
       isManifestBacked: vi.fn().mockResolvedValue(false),
       listManifestSourceNames: vi.fn().mockResolvedValue([]),
       loadSource: vi.fn().mockResolvedValue(null),
-      loadAllSources: vi.fn().mockResolvedValue([]),
+      loadAllSources: vi.fn().mockResolvedValue({ sources: [], loadErrors: [] }),
       validatePhysicalTableReferences: vi.fn().mockResolvedValue([]),
     } as never,
     connections: {