feat(client): render province boundaries on planetary map (#927, D-205)

Export pre-computed drainage basin boundaries from atlas_province_boundaries
into markers.json during generate_atlas. AtlasMarkerOverlay draws province
polylines with semi-transparent fill under the political_zones overlay.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tooling/planet-gen/generate_atlas.py

@@ -1162,6 +1162,7 @@ def load_markers(body_dir: Path) -> dict:
         "cities": [],
         "railroads": [],
         "pois": [],
+        "provinces": [],
     }
 
 
@@ -1310,6 +1311,38 @@ def process_body(
     return {"status": "generated", "markers": markers}
 
 
+def query_province_boundaries(conn: sqlite3.Connection, body_id: str) -> list[dict]:
+    """Read pre-computed province boundaries for a body from atlas_province_boundaries."""
+    rows = conn.execute(
+        "SELECT basin_id, path, area_pct FROM atlas_province_boundaries WHERE body_id = ? ORDER BY basin_id",
+        (body_id,),
+    ).fetchall()
+    provinces = []
+    for row in rows:
+        provinces.append({
+            "basin_id": row[0],
+            "path": json.loads(row[1]),
+            "area_pct": row[2],
+        })
+    return provinces
+
+
+def inject_provinces_into_markers(conn: sqlite3.Connection, body_id: str, body_dir: Path) -> bool:
+    """Add provinces array to an existing markers.json. Returns True if written."""
+    markers_path = body_dir / "markers.json"
+    if not markers_path.exists():
+        return False
+    provinces = query_province_boundaries(conn, body_id)
+    if not provinces:
+        return False
+    with open(markers_path) as f:
+        markers = json.load(f)
+    markers["provinces"] = provinces
+    with open(markers_path, "w") as f:
+        json.dump(markers, f, indent=2)
+    return True
+
+
 def main():
     parser = argparse.ArgumentParser(
         description="Atlas generation — terrain-aware city placement and infrastructure (#832)"
@@ -1406,6 +1439,18 @@ def main():
             n_errors += 1
             print(f"  [{i+1}/{len(bodies)}] {body_id:20s} ERROR: {result.get('message', '')}")
 
+    # Inject province boundaries into markers.json for bodies that have them
+    if not args.dry_run:
+        n_provinces = 0
+        for body_info in bodies:
+            body_id = body_info["body_id"]
+            terrain_ref = body_info["terrain_reference"]
+            body_dir = REPO_ROOT / Path(terrain_ref).parent
+            if inject_provinces_into_markers(conn, body_id, body_dir):
+                n_provinces += 1
+        if n_provinces > 0:
+            print(f"\n  Province boundaries injected into {n_provinces} markers.json files.")
+
     if not args.dry_run:
         # Stamp generator metadata (#855, #856) together with the atlas data
         # in a single commit — atlas data + stamp land atomically, and the