Perf clusters (#3127)

## Description:

This PR reduces server/client tick CPU spent on territory cluster
maintenance by:
- Cutting redundant work in `PlayerExecution.removeClusters()`
(largest-cluster bounding box reuse, fewer allocations in
`isSurrounded()` and `removeCluster()`).
- Making `calculateBoundingBox()` allocation-free per tile by switching
from `gm.cell(tile)` to `gm.x(tile)`/`gm.y(tile)` (it now only allocates
the two result `Cell`s, instead of 1 per tile).

## Commits
- `51de0a1b` core: reduce PlayerExecution cluster overhead
- `6d9d85c5` core: avoid Cell allocations in PlayerExecution
isSurrounded
- `346f6a8c` core(util): speed up calculateBoundingBox by avoiding Cell
allocations


## Notes
- This PR is intended to be behavior-preserving; changes are limited to
hot-path micro-optimizations.
- Follow-up opportunity: `calculateClusters`/flood-fill is now the top
hotspot; further wins likely come from reducing traversal work or
caching.


## Please complete the following:

- [ ] I have added screenshots for all UI updates
- [ ] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [ ] I have added relevant tests to the test directory
- [ ] 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:

DISCORD_USERNAME

src/core/Util.ts

@@ -80,13 +80,14 @@ export function calculateBoundingBox(
     maxX = -Infinity,
     maxY = -Infinity;
 
-  borderTiles.forEach((tile: TileRef) => {
-    const cell = gm.cell(tile);
-    minX = Math.min(minX, cell.x);
-    minY = Math.min(minY, cell.y);
-    maxX = Math.max(maxX, cell.x);
-    maxY = Math.max(maxY, cell.y);
-  });
+  for (const tile of borderTiles) {
+    const x = gm.x(tile);
+    const y = gm.y(tile);
+    minX = Math.min(minX, x);
+    minY = Math.min(minY, y);
+    maxX = Math.max(maxX, x);
+    maxY = Math.max(maxY, y);
+  }
 
   return { min: new Cell(minX, minY), max: new Cell(maxX, maxY) };
 }