scope frontend contracts to the feature, not to one file

93 test files read a single frontend source and asserted identifiers in it. The
MapWorkspace split showed what that costs: 24 tests went red for a move that
changed no behaviour at all. A contract belongs to the feature — a container,
its hooks, its domain layer — not to whichever file currently holds it.

232 read sites now resolve through read_feature(). The distinction that makes
this safe is direction: a *positive* contract ("this is wired") may widen,
because the identifier must still exist somewhere in the feature; a *negative*
one ("this component performs no transport") is a statement about one file, and
widening it would quietly weaken the check. The 73 single-file reads that
remain are exactly those, and a guard now enforces the rule for new tests.

Verified rather than assumed: of the 732 migrated positive assertions, 644 still
match exactly one module — as specific as before — and the other 86 already
spanned a container and its hook by nature. Two apparent misses are an artefact
of the checking regex reading an escaped newline literally.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/tests/test_sprint88_ai_lab_density.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 from pathlib import Path
+from tests.frontend_contract import read_feature
 
 
 ROOT = Path(__file__).resolve().parents[2]
@@ -27,9 +28,7 @@ def test_detection_lab_exposes_structured_surfaces() -> None:
 
 
 def test_segmentation_lab_exposes_structured_surfaces() -> None:
-    lab = (ROOT / "frontend" / "src" / "components" / "segmentation" / "SegmentationLab.tsx").read_text(
-        encoding="utf-8"
-    )
+    lab = read_feature("segmentation")
 
     assert 'className="workspace-panel ai-lab-shell segmentation-lab-shell"' in lab
     assert 'className="ai-lab-model-surface"' in lab
@@ -44,12 +43,8 @@ def test_segmentation_lab_exposes_structured_surfaces() -> None:
 
 
 def test_ai_lab_preserves_existing_detection_and_segmentation_controls() -> None:
-    detection = (ROOT / "frontend" / "src" / "components" / "detection" / "DetectionLab.tsx").read_text(
-        encoding="utf-8"
-    )
-    segmentation = (ROOT / "frontend" / "src" / "components" / "segmentation" / "SegmentationLab.tsx").read_text(
-        encoding="utf-8"
-    )
+    detection = read_feature("detection")
+    segmentation = read_feature("segmentation")
 
     assert "onRunDetection" in detection
     assert "onLoadResults" in detection