extract the map workspace's domain layer out of the component

MapWorkspace.tsx opened with ~590 lines of theme catalogue, dataset matching
and label formatting above a 3.200-line component. None of it is React, all of
it is independently testable, and both render paths read from it, so it belongs
beside the pure helpers that already live in mapWorkspaceUtils.

The contract tests that read MapWorkspace.tsx would have gone red for a move
that changes no behaviour at all — 24 of them. That is the brittleness the
frontend_contract helper exists to remove, so it gains read_map_workspace():
the workspace is one feature spread over several modules, and a contract
belongs to the feature rather than to whichever file currently holds it.

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

backend/tests/test_sprint223_governed_grb_refresh.py

@@ -13,6 +13,7 @@ import pytest
 from app.core.errors import AppError
 from app.models import Dataset
 from app.services.grb_refresh_plan_service import GrbRefreshPlanService
+from tests.frontend_contract import read_map_workspace
 
 
 NOW = datetime(2026, 7, 16, 16, 0, tzinfo=timezone.utc)
@@ -252,7 +253,7 @@ def test_refresh_api_and_frontend_remain_explicit_only() -> None:
 
 def test_map_theme_ranking_prefers_newer_observation_over_feature_count() -> None:
     root = Path(__file__).resolve().parents[2]
-    workspace = (root / "frontend" / "src" / "components" / "map" / "MapWorkspace.tsx").read_text(encoding="utf-8")
+    workspace = read_map_workspace()
     observed_sort = workspace.index("const observedAtDifference")
     feature_tiebreaker = workspace.index("right.feature_count", observed_sort)
     assert observed_sort < feature_tiebreaker
@@ -262,7 +263,7 @@ def test_map_theme_ranking_prefers_newer_observation_over_feature_count() -> Non
 
 def test_map_workspace_restores_theme_from_selected_dataset() -> None:
     root = Path(__file__).resolve().parents[2]
-    workspace = (root / "frontend" / "src" / "components" / "map" / "MapWorkspace.tsx").read_text(encoding="utf-8")
+    workspace = read_map_workspace()
     assert "function themeIdForDataset(" in workspace
     assert "useState<DataThemeId>(() =>" in workspace
     assert "return themeIdForDataset(selectedDataset) ?? 'buildings'" in workspace