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.

python/ktx-sl/semantic_layer/__main__.py

@@ -1,3 +1,22 @@
-from semantic_layer.cli import main
+from __future__ import annotations
 
-main()
+import json
+import sys
+
+from semantic_layer.cli import main as cli_main
+from semantic_layer.models import SourceDefinition
+
+
+def dump_schema() -> None:
+    json.dump(
+        SourceDefinition.model_json_schema(), sys.stdout, indent=2, sort_keys=True
+    )
+    sys.stdout.write("\n")
+
+
+if __name__ == "__main__":
+    if len(sys.argv) > 1 and sys.argv[1] in {"dump-schema", "schema"}:
+        sys.argv.pop(1)
+        dump_schema()
+    else:
+        cli_main()