feat(formdsl): add DGSEM multipatch compatibility diagnostics

docs/exec-plans/active/20260407-formdsl-dgsem-multipatch-compatibility-semantics.md

@@ -30,9 +30,9 @@ keeping DG-SEM multipatch execution unsupported.
 ## Implementation Checklist
 
 - [x] Add OpenSpec proposal/design/spec/tasks artifacts.
-- [ ] Implement DG-SEM compatibility diagnostics for multipatch descriptors.
-- [ ] Add strict/permissive regression tests.
-- [ ] Update docs and progress tracking.
+- [x] Implement DG-SEM compatibility diagnostics for multipatch descriptors.
+- [x] Add strict/permissive regression tests.
+- [x] Update docs and progress tracking.
 
 ## Risks / Open Questions
 

docs/ufl-backend-support.md

@@ -115,6 +115,12 @@ Common diagnostic codes:
 - `unsupported_value_shape`
 - `unsupported_multipatch_interface`
 
+DG-SEM multipatch compatibility-only diagnostics (permissive mode):
+
+- `dgsem_multipatch_compatibility_profile`
+- `dgsem_unsupported_multipatch_orientation`
+- `dgsem_unsupported_multipatch_penalty_control`
+
 ## DG-SEM Prerequisite
 
 The `dgsem` backend requires an explicit `MeshmodeCutOverlay` payload from

openspec/changes/formdsl-dgsem-multipatch-compatibility-semantics/tasks.md

@@ -5,12 +5,12 @@
 
 ## 2. Capability Diagnostics
 
-- [ ] 2.1 Keep strict unsupported behavior and add deterministic compatibility
+- [x] 2.1 Keep strict unsupported behavior and add deterministic compatibility
   profile diagnostics in permissive mode.
-- [ ] 2.2 Emit explicit DG-SEM diagnostics for orientation variants and
+- [x] 2.2 Emit explicit DG-SEM diagnostics for orientation variants and
   per-interface penalty controls.
 
 ## 3. Validation + Docs
 
-- [ ] 3.1 Add regression tests for strict/permissive diagnostics stability.
-- [ ] 3.2 Update docs and execution-plan progress.
+- [x] 3.1 Add regression tests for strict/permissive diagnostics stability.
+- [x] 3.2 Update docs and execution-plan progress.

src/cutkit/formdsl/capabilities.py

@@ -27,6 +27,60 @@
 }
 
 
+def _dgsem_multipatch_compatibility_diagnostics(
+    form_ir: WeakFormIR,
+) -> tuple[CapabilityDiagnostic, ...]:
+    multipatch = form_ir.multipatch
+    if multipatch is None:
+        return ()
+
+    diagnostics: list[CapabilityDiagnostic] = [
+        CapabilityDiagnostic(
+            code="dgsem_multipatch_compatibility_profile",
+            backend="dgsem",
+            detail=(
+                "dgsem multipatch compatibility is diagnostics-only "
+                f"(patches={len(multipatch.patch_ids)}, interfaces={len(multipatch.interfaces)})"
+            ),
+        )
+    ]
+
+    reversed_count = sum(
+        1 for interface in multipatch.interfaces if interface.orientation == "reversed"
+    )
+    if reversed_count > 0:
+        diagnostics.append(
+            CapabilityDiagnostic(
+                code="dgsem_unsupported_multipatch_orientation",
+                backend="dgsem",
+                detail=(
+                    "dgsem multipatch compatibility does not support "
+                    f"reversed orientation interfaces (count={reversed_count})"
+                ),
+                alternatives=("aligned",),
+            )
+        )
+
+    controlled_penalties = sum(
+        1 for interface in multipatch.interfaces if interface.penalty is not None
+    )
+    if controlled_penalties > 0:
+        diagnostics.append(
+            CapabilityDiagnostic(
+                code="dgsem_unsupported_multipatch_penalty_control",
+                backend="dgsem",
+                detail=(
+                    "dgsem multipatch compatibility does not support "
+                    "per-interface penalty controls "
+                    f"(count={controlled_penalties})"
+                ),
+                alternatives=("multipatch_penalty",),
+            )
+        )
+
+    return tuple(diagnostics)
+
+
 def _is_supported_value_shape(value_shape: tuple[int, ...], *, backend: str) -> bool:
     if value_shape == ():
         return True
@@ -155,6 +209,8 @@ def check_support(
                 alternatives=alternatives,
             )
         )
+        if backend == "dgsem":
+            diagnostics.extend(_dgsem_multipatch_compatibility_diagnostics(form_ir))
 
     if strict and diagnostics:
         raise CapabilityError(diagnostics[0])

tests/formdsl/test_formdsl_assembly.py

@@ -1529,7 +1529,70 @@ def test_dgsem_permissive_reports_multipatch_interface_diagnostic() -> None:
         strict=False,
     )
 
-    assert any(d.code == "unsupported_multipatch_interface" for d in result.diagnostics)
+    codes = {d.code for d in result.diagnostics}
+    assert "unsupported_multipatch_interface" in codes
+    assert "dgsem_multipatch_compatibility_profile" in codes
+
+
+def test_dgsem_permissive_reports_reversed_orientation_compatibility_diagnostic() -> (
+    None
+):
+    form = _base_form()
+    form["multipatch"] = {
+        "patch_ids": ["patch-a", "patch-b"],
+        "interfaces": [
+            {
+                "plus_patch": "patch-b",
+                "minus_patch": "patch-a",
+                "plus_boundary": "right",
+                "minus_boundary": "top",
+                "orientation": "reversed",
+            }
+        ],
+    }
+
+    result = assemble_form(
+        form,
+        backend="dgsem",
+        overlay_payload=_overlay_contract(),
+        strict=False,
+    )
+
+    diagnostics_by_code = {d.code: d for d in result.diagnostics}
+    assert "dgsem_unsupported_multipatch_orientation" in diagnostics_by_code
+    assert diagnostics_by_code[
+        "dgsem_unsupported_multipatch_orientation"
+    ].alternatives == ("aligned",)
+
+
+def test_dgsem_permissive_reports_penalty_control_compatibility_diagnostic() -> None:
+    form = _base_form()
+    form["multipatch"] = {
+        "patch_ids": ["patch-a", "patch-b"],
+        "interfaces": [
+            {
+                "plus_patch": "patch-b",
+                "minus_patch": "patch-a",
+                "plus_boundary": "right",
+                "minus_boundary": "top",
+                "orientation": "aligned",
+                "penalty": 1.5,
+            }
+        ],
+    }
+
+    result = assemble_form(
+        form,
+        backend="dgsem",
+        overlay_payload=_overlay_contract(),
+        strict=False,
+    )
+
+    diagnostics_by_code = {d.code: d for d in result.diagnostics}
+    assert "dgsem_unsupported_multipatch_penalty_control" in diagnostics_by_code
+    assert diagnostics_by_code[
+        "dgsem_unsupported_multipatch_penalty_control"
+    ].alternatives == ("multipatch_penalty",)
 
 
 def test_iga_rejects_vector_value_shape() -> None: