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_sprint62_frontend_visual_polish.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]
@@ -11,14 +12,7 @@ def test_frontend_shell_has_atlas_workbench_contracts() -> None:
     css = (ROOT / "frontend" / "src" / "styles" / "atlas-workbench.css").read_text(
         encoding="utf-8"
     )
-    navigation = (
-        ROOT
-        / "frontend"
-        / "src"
-        / "components"
-        / "shell"
-        / "WorkbenchNavigation.tsx"
-    ).read_text(encoding="utf-8")
+    navigation = read_feature("shell")
 
     assert "workspace-command-bar" not in app
     assert "workspace-nav-cluster" not in app
@@ -36,15 +30,9 @@ def test_primary_panels_use_empty_state_components() -> None:
     project_panel = (ROOT / "frontend" / "src" / "components" / "project" / "ProjectPanel.tsx").read_text(
         encoding="utf-8"
     )
-    dataset_panel = (ROOT / "frontend" / "src" / "components" / "datasets" / "DatasetPanel.tsx").read_text(
-        encoding="utf-8"
-    )
-    detection_lab = (ROOT / "frontend" / "src" / "components" / "detection" / "DetectionLab.tsx").read_text(
-        encoding="utf-8"
-    )
-    segmentation_lab = (
-        ROOT / "frontend" / "src" / "components" / "segmentation" / "SegmentationLab.tsx"
-    ).read_text(encoding="utf-8")
+    dataset_panel = read_feature("datasets")
+    detection_lab = read_feature("detection")
+    segmentation_lab = read_feature("segmentation")
 
     assert "empty-state" in project_panel
     assert "empty-state" in dataset_panel