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_sprint183_map_layer_source_mode.py

@@ -1,12 +1,13 @@
 from pathlib import Path
+from tests.frontend_contract import read_feature
 
 
 ROOT = Path(__file__).resolve().parents[2]
 
 
 def test_map_source_mode_keeps_database_layers_distinct_from_analysis_results() -> None:
-    app = (ROOT / "frontend/src/App.tsx").read_text(encoding="utf-8")
-    workspace = (ROOT / "frontend/src/components/map/MapWorkspace.tsx").read_text(encoding="utf-8")
+    app = read_feature("shell")
+    workspace = read_feature("map_workspace")
 
     assert "const [mapContentMode, setMapContentMode]" in app
     assert "mapContentMode === 'analysis' && analysisMapLayerAvailable" in app