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_sprint53_selection_ergonomics.py

@@ -1,13 +1,12 @@
 from pathlib import Path
+from tests.frontend_contract import read_feature
 
 
 ROOT = Path(__file__).resolve().parents[2]
 
 
 def test_dataset_panel_exposes_map_and_export_quick_actions() -> None:
-    panel = (ROOT / "frontend" / "src" / "components" / "datasets" / "DatasetPanel.tsx").read_text(
-        encoding="utf-8"
-    )
+    panel = read_feature("datasets")
 
     assert "selectedDatasetId" in panel
     assert "dataset-card-active" in panel
@@ -29,7 +28,7 @@ def test_data_workspace_keeps_catalog_wide_enough_for_populated_state() -> None:
 
 def test_shell_preserves_workspace_width_on_standard_desktop_viewports() -> None:
     css = (ROOT / "frontend" / "src" / "styles" / "app.css").read_text(encoding="utf-8")
-    app = (ROOT / "frontend" / "src" / "App.tsx").read_text(encoding="utf-8")
+    app = read_feature("shell")
 
     assert "grid-template-columns: 12.5rem minmax(0, 1fr) 21rem" in css
     assert "@media (max-width: 1360px)" in css
@@ -40,7 +39,7 @@ def test_shell_preserves_workspace_width_on_standard_desktop_viewports() -> None
 
 
 def test_app_wires_dataset_quick_actions_to_existing_workspaces() -> None:
-    app = (ROOT / "frontend" / "src" / "App.tsx").read_text(encoding="utf-8")
+    app = read_feature("shell")
 
     assert "const openDatasetInMap = (dataset: DatasetCreateResponse) => {" in app
     assert "loadDatasetDetails(selectedProjectId, dataset)" in app