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/tests/test_loader.py

@@ -148,11 +148,21 @@ class TestLoaderEdgeCases:
             with open(Path(tmpdir) / "test.yaml", "w") as f:
                 yaml.dump(data, f)
             loader = SourceLoader(tmpdir)
-            try:
-                sources = loader.load_all()
-                assert "test" in sources
-            except Exception:
-                pass
+            with pytest.raises(Exception, match="unknown_field"):
+                loader.load_all()
+
+    def test_source_requires_table_or_sql(self):
+        with tempfile.TemporaryDirectory() as tmpdir:
+            data = {
+                "name": "test",
+                "grain": ["id"],
+                "columns": [{"name": "id", "type": "number"}],
+            }
+            with open(Path(tmpdir) / "test.yaml", "w") as f:
+                yaml.dump(data, f)
+            loader = SourceLoader(tmpdir)
+            with pytest.raises(Exception, match="table.*sql"):
+                loader.load_file(Path(tmpdir) / "test.yaml")
 
     def test_subdirectory_sources(self):
         with tempfile.TemporaryDirectory() as tmpdir:

python/ktx-sl/tests/test_manifest.py

@@ -205,12 +205,15 @@ class TestValidateOverlay:
             "descriptions": {"user": "Revenue-bearing orders"},
             "grain": ["id"],
             "measures": [{"name": "revenue", "expr": "sum(total)"}],
+            "column_overrides": [
+                {"name": "status", "descriptions": {"user": "Order lifecycle status"}}
+            ],
             "columns": [
                 {"name": "is_high_value", "expr": "total > 1000", "type": "boolean"}
             ],
             "exclude_columns": ["status"],
         }
-        errors = validate_overlay(data)
+        errors = validate_overlay(data, {"status", "total"})
         assert errors == []
 
     def test_validate_overlay_rejects_table(self):
@@ -225,14 +228,13 @@ class TestValidateOverlay:
         assert len(errors) == 1
         assert "sql" in errors[0].lower()
 
-    def test_validate_overlay_rejects_type_without_expr(self):
+    def test_validate_overlay_rejects_column_without_expr(self):
         data = {
             "name": "orders",
             "columns": [{"name": "status", "type": "string"}],
         }
         errors = validate_overlay(data)
         assert len(errors) == 1
-        assert "type" in errors[0].lower()
         assert "expr" in errors[0].lower()
 
     def test_validate_overlay_allows_type_with_expr(self):
@@ -243,6 +245,33 @@ class TestValidateOverlay:
         errors = validate_overlay(data)
         assert errors == []
 
+    def test_validate_overlay_rejects_column_override_structural_fields(self):
+        data = {
+            "name": "orders",
+            "column_overrides": [
+                {
+                    "name": "status",
+                    "description": "Status",
+                    "type": "string",
+                    "expr": "status",
+                }
+            ],
+        }
+        errors = validate_overlay(data, {"status"})
+        assert len(errors) == 3
+        assert "descriptions" in errors[0]
+        assert "type" in errors[1]
+        assert "expr" in errors[2]
+
+    def test_validate_overlay_rejects_unknown_column_override(self):
+        data = {
+            "name": "orders",
+            "column_overrides": [{"name": "missing", "descriptions": {"user": "Nope"}}],
+        }
+        errors = validate_overlay(data, {"status"})
+        assert len(errors) == 1
+        assert "does not match" in errors[0]
+
 
 # ── Two-Tier Loading Tests ─────────────────────────────────────────
 
@@ -502,6 +531,77 @@ class TestTwoTierLoading:
         assert hv.expr == "total > 1000"
         assert hv.type == "boolean"
 
+    def test_overlay_column_overrides_patch_manifest_columns(self, tmp_path: Path):
+        schema_dir = tmp_path / "_schema"
+        _write_yaml(schema_dir / "public.yaml", _manifest_tables())
+
+        overlay = {
+            "name": "orders",
+            "column_overrides": [
+                {"name": "status", "descriptions": {"user": "Order lifecycle status"}}
+            ],
+        }
+        _write_yaml(tmp_path / "orders.yaml", overlay)
+        _write_yaml(tmp_path / "customers.yaml", {"name": "customers"})
+
+        loader = SourceLoader(tmp_path)
+        sources = loader.load_all()
+
+        status = next(c for c in sources["orders"].columns if c.name == "status")
+        assert status.type == "string"
+        assert status.description == "Order lifecycle status"
+        assert status.descriptions == {"user": "Order lifecycle status"}
+
+    def test_overlay_rejects_unknown_column_override(self, tmp_path: Path):
+        schema_dir = tmp_path / "_schema"
+        _write_yaml(schema_dir / "public.yaml", _manifest_tables())
+
+        overlay = {
+            "name": "orders",
+            "column_overrides": [
+                {"name": "missing", "descriptions": {"user": "No such column"}}
+            ],
+        }
+        _write_yaml(tmp_path / "orders.yaml", overlay)
+        _write_yaml(tmp_path / "customers.yaml", {"name": "customers"})
+
+        loader = SourceLoader(tmp_path)
+        with pytest.raises(ValueError, match="Column override 'missing'"):
+            loader.load_all()
+
+    def test_overlay_rejects_computed_column_name_collision(self, tmp_path: Path):
+        schema_dir = tmp_path / "_schema"
+        _write_yaml(schema_dir / "public.yaml", _manifest_tables())
+
+        overlay = {
+            "name": "orders",
+            "columns": [{"name": "status", "type": "string", "expr": "status"}],
+        }
+        _write_yaml(tmp_path / "orders.yaml", overlay)
+        _write_yaml(tmp_path / "customers.yaml", {"name": "customers"})
+
+        loader = SourceLoader(tmp_path)
+        with pytest.raises(ValueError, match="move it to 'column_overrides:'"):
+            loader.load_all()
+
+    def test_overlay_rejects_exclude_override_conflict(self, tmp_path: Path):
+        schema_dir = tmp_path / "_schema"
+        _write_yaml(schema_dir / "public.yaml", _manifest_tables())
+
+        overlay = {
+            "name": "orders",
+            "exclude_columns": ["status"],
+            "column_overrides": [
+                {"name": "status", "descriptions": {"user": "Hidden status"}}
+            ],
+        }
+        _write_yaml(tmp_path / "orders.yaml", overlay)
+        _write_yaml(tmp_path / "customers.yaml", {"name": "customers"})
+
+        loader = SourceLoader(tmp_path)
+        with pytest.raises(ValueError, match="conflict with exclude_columns"):
+            loader.load_all()
+
     def test_overlay_measures_set(self, tmp_path: Path):
         schema_dir = tmp_path / "_schema"
         _write_yaml(schema_dir / "public.yaml", _manifest_tables())