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_sprint84_data_workspace_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]
@@ -25,9 +26,7 @@ def test_project_and_area_panels_expose_selected_summary_regions() -> None:
 
 
 def test_dataset_panel_separates_selected_upload_and_catalog_regions() -> None:
-    dataset_panel = (ROOT / "frontend" / "src" / "components" / "datasets" / "DatasetPanel.tsx").read_text(
-        encoding="utf-8"
-    )
+    dataset_panel = read_feature("datasets")
 
     assert "const selectedDataset = datasets.find" in dataset_panel
     assert 'className="data-selection-summary data-selection-summary-dataset"' in dataset_panel