fix(task-21): the applied ISO layer set is per document — it follows document_activate on ezdxf and is cleared on every document switch on both engines

`drawing_apply_iso_layers` recorded its standard in `_active_layer_set` on
the backend, and `_role_layer` reads it ahead of the PlanSpec to decide where
`dimension_auto` / `construction_xline` file their output. It was neither a
`_DocState` field nor cleared by `_reset_document_state`, so an iso13567
sheet next to a mech one pushed the mech drawing's dimensions onto
`M-DIMEN-T-N` (created on demand), silently — and `drawing_critique` could
not see it because the same stale set drove the critique. The attribute
predates the registry (v1.5 `drawing_new` did not reset it either), but the
per-document claim of Task 21 covered it and the measured attribute list
missed it.

- `_DocState.active_layer_set` + `_ActiveField("active_layer_set")` on
  `EzdxfBackend`: the set follows the active entry, bidirectionally.
- `PremiumContract._reset_document_state` clears it (declared on the mixin
  like `_plan_spec`), which fixes COM's `_ensure_document_state` switch and
  `drawing_new` / `drawing_open` / `disconnect` on both engines.
- tests: `test_active_layer_set_is_per_document` (headless registry, the
  reviewer's repro plus the reverse direction) and the COM activate test now
  also asserts the set is cleared and `_role_layer('dim')` is back on DIM.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd

backends/contracts/premium.py

@@ -36,11 +36,18 @@ class PremiumContract(ABC):
 
     _plan_spec: PlanSpec | None = None
     _preflight_result: Any = None
+    # The standard `drawing_apply_iso_layers` last bootstrapped on *this*
+    # drawing (None until it runs) — `_active_layer_set_id` reads it ahead of
+    # the PlanSpec. Drawing-scoped like the plan: a live backend clears it
+    # when AutoCAD's active document changes, the headless registry keeps one
+    # per open document.
+    _active_layer_set: LayerSetId | None = None
 
     def _reset_document_state(self) -> None:
         """Clear metadata that is valid only for the current drawing."""
         self._plan_spec = None
         self._preflight_result = None
+        self._active_layer_set = None
         self._gdt_datums_defined = set()
         self._gdt_datums_referenced = set()
 

backends/ezdxf_backend.py

@@ -977,6 +977,11 @@ class _DocState:
     transaction_stack: list[Path] = field(default_factory=list)
     plan_spec: Any = None
     preflight_result: Any = None
+    # `drawing_apply_iso_layers` records the standard here so `_role_layer`
+    # files dims / scaffolding on that standard's layer; it is per drawing,
+    # not per backend (an iso13567 sheet next to a mech one must not push
+    # the mech drawing's dimensions onto M-DIMEN-T-N).
+    active_layer_set: str | None = None
     gdt_datums_defined: set = field(default_factory=set)
     gdt_datums_referenced: set = field(default_factory=set)
     activated_seq: int = 0
@@ -1033,6 +1038,7 @@ class EzdxfBackend(AutoCADBackend):
     _transaction_stack = _ActiveField("transaction_stack")
     _plan_spec = _ActiveField("plan_spec")
     _preflight_result = _ActiveField("preflight_result")
+    _active_layer_set = _ActiveField("active_layer_set")
     _gdt_datums_defined = _ActiveField("gdt_datums_defined")
     _gdt_datums_referenced = _ActiveField("gdt_datums_referenced")