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_sprint219_regional_raster_explorer.py

@@ -1,5 +1,5 @@
 from pathlib import Path
-from tests.frontend_contract import assert_calls, assert_mentions, assert_wired
+from tests.frontend_contract import assert_calls, assert_mentions, assert_wired, read_map_workspace
 
 
 ROOT = Path(__file__).resolve().parents[2]
@@ -21,7 +21,7 @@ def test_partitioned_raster_routes_are_canonical_and_documented() -> None:
 
 def test_regional_map_uses_logical_partition_groups_and_exact_analysis() -> 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")
+    workspace = read_map_workspace()
     hook = (ROOT / "frontend/src/hooks/useMapThemeSelectionInsights.ts").read_text(encoding="utf-8")
     api = (ROOT / "frontend/src/services/api/datasets.ts").read_text(encoding="utf-8")