Cut worker→main bandwidth ~3.3× by switching PlayerUpdate to deltas (#3967)

## Description:

Cut worker→main bandwidth ~3.3× by switching PlayerUpdate from a full
per-tick snapshot to a field-level diff. PlayerImpl.toUpdate() now
caches the last sent update and returns only changed fields, or null if
nothing changed. The client-side applyStateUpdate() merges instead of
overwriting.

Per-tick total dropped from ~297 KB to ~89 KB; the Player bucket alone
went from 258 KB/tick to 50 KB/tick. Diff/apply logic lives in a new
GameUpdateUtils.ts module with unit tests.

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Please put your Discord username so you can be contacted if a bug or
regression is found:

evan

tests/Disconnected.test.ts

@@ -80,7 +80,7 @@ describe("Disconnected", () => {
     test("should include disconnected state in player update", () => {
       player1.markDisconnected(true);
       const update = player1.toUpdate();
-      expect(update.isDisconnected).toBe(true);
+      expect(update?.isDisconnected).toBe(true);
     });
   });
 
@@ -153,8 +153,9 @@ describe("Disconnected", () => {
     test("should maintain disconnected state in player updates across ticks", () => {
       player1.markDisconnected(true);
       executeTicks(game, 3);
-      const update = player1.toUpdate();
-      expect(update.isDisconnected).toBe(true);
+      // toUpdate() returns diffs after the first call, so query engine state
+      // directly rather than the wire payload (which only carries changed fields).
+      expect(player1.isDisconnected()).toBe(true);
     });
   });