Add station listing and cargo queries to admin client

Extends the AdminBridge GameScript JSON channel (the same relay used by
list_vehicles/get_timetable) with station support:

- list_stations(): enumerate stations, fire-and-forget like list_vehicles().
- get_station(): authoritative per-cargo snapshot of a station's live state,
  with both the real-time waiting amount (GSStation.GetCargoWaiting) and the
  planned cargodist link-graph flow (GetCargoPlanned), plus rating.
- get_station_cargo(): break one cargo type down by source station and by
  next hop (the cargodist routing destination) for both waiting and planned
  amounts, with optional from_station/via_station filters.

All three use stock GameScript API (no server patch, unlike timetables).
Refactors the shared GS request/reply correlation out of get_timetable and
get_station into a _gs_query() helper. Companion handlers must be added to
the server-side AdminBridge GameScript (not tracked in this repo).

Includes unit + e2e tests, a worked demo in main_admin.py, and protocol docs.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

main_admin.py

@@ -49,7 +49,42 @@ async def run_admin():
 
         print("--- Requesting vehicle info via GameScript ---")
         await admin.list_vehicles()
-        
+
+        # Capture station-list replies (delivered to on_gamescript, like list_vehicles) while
+        # still logging every other GameScript message.
+        stations = []
+        def gamescript_capture(data):
+            if isinstance(data, dict) and "stations" in data:
+                stations.append(data["stations"])
+            gamescript_logger(data)
+        admin.on_gamescript = gamescript_capture
+
+        print("--- Requesting station info via GameScript ---")
+        await admin.list_stations()
+        await asyncio.sleep(1)
+
+        # Fetch one station's authoritative live cargo (real-time waiting + planned).
+        if stations and stations[-1]:
+            sid = stations[-1][0]["id"]
+            try:
+                data = await admin.get_station(sid, timeout=10.0)
+                print(f"--- Station {sid} ({data.get('name')}) cargo: real-time waiting vs planned ---")
+                for cargo in data.get("cargo", []):
+                    print(f"    cargo {cargo['cargo_id']}: waiting={cargo['waiting']} "
+                          f"planned={cargo['planned']} rating={cargo['rating']}")
+
+                # Break the first cargo down by source station and by next hop (routing destination).
+                if data.get("cargo"):
+                    cid = data["cargo"][0]["cargo_id"]
+                    flow = await admin.get_station_cargo(sid, cid, timeout=10.0)
+                    print(f"--- Station {sid} cargo {cid} flow breakdown (station 65535 = none/deleted) ---")
+                    print(f"    waiting by source: {flow['waiting_by_from']}")
+                    print(f"    waiting by next hop: {flow['waiting_by_via']}")
+                    print(f"    planned by source: {flow['planned_by_from']}")
+                    print(f"    planned by next hop: {flow['planned_by_via']}")
+            except Exception as e:
+                print(f"!!! station query failed: {e}")
+
         await asyncio.sleep(5)
         print("--- Quitting ---")
         await admin.quit()