Let a tile report a touch instead of blocking it (#160)

A tile could be solid, one-way and scaled, but not report without
blocking. Water, lava, spikes or a goal zone needed a trigger object
of their own laid over the map. Now a tile has a switch Trigger, and
the collision events name the tile and the cell they met.

- A tile has the switch "Trigger (reports, never blocks)" under
  Solid. Its cells hold nothing up: no Move, no falling body, no
  ray. They raise On collision enter, stay and exit with the tilemap
  as other.
- The three events have the outputs tile, cell x and cell y. Without
  a tile, tile is empty and the cell is -1, -1.
- The three events have the input per cell. Off, the default, gives
  one event per area: cells of the same tile that touch side by
  side. On gives one event per cell.
- The scene view outlines trigger cells in yellow like a trigger
  collider, merged to rectangles but never with solid cells.
- Is touching with the map counts trigger tiles, and Set tile works
  while the game runs.
- Both systems find the touches with the same overlap check in
  runtime/systems/tileTriggers.ts, not through the physics engine.

Decided here:

- Trigger wins over Solid. A tile with both does not block, as with
  a collider.
- Water beside lava is two areas, and so are two ponds that touch
  only at a corner.
- An area names the cell nearest to the centre of the collider, and
  on exit the cell it named last.
- The solid cells of a map still do not name their tile.
- Normal, impulse and point are 0, as with a trigger collider.
- Collision scale and outlines shape a trigger cell like a solid
  one. One-way, friction and bounciness have no effect.

Tests:
- Gate on stein2: 205 files, 3104 tests (3067 + 37), exit 0
- e2e 39,40: 106 checks. e2e 1-8,39-43: 490 checks. Both on
  melektaus-atx. No full run, the sum of 1648 (1645 + 3) is
  reckoned and CI measures it.
- 37 unit tests in tests/core/tile-triggers.test.ts, nearly all
  against both systems
- Three e2e checks with a real mouse in block 40l-iv
- 24 unit counter-probes fail tests. Two did not at first, tests
  were added for them.
- Two browser counter-probes fail their checks
- Two screenshots looked at (yellow outline, the node with per cell)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/runtime/systems/solids.ts

@@ -72,6 +72,26 @@ export interface CollisionPair {
   normal?: Vec2;
   impulse?: number;
   point?: Vec2;
+  /** Set where `b` is a map and `a` overlaps a trigger tile of it (#160) – a pair of its own, next to the map's. */
+  tile?: TileContact;
+}
+
+/**
+ * Which trigger tile of a map a collider overlaps (#160). A pair stands either for one cell or for a
+ * whole area of the tile; an area names the cell of it nearest to the collider.
+ */
+export interface TileContact {
+  tileId: string;
+  cell: { x: number; y: number };
+  area: string;
+  perCell: boolean;
+}
+
+/** What a pair is known by from one tick to the next. */
+export function pairKey({ a, b, tile }: CollisionPair): string {
+  const key = `${a.id}|${b.id}`;
+  if (!tile) return key;
+  return `${key}|${tile.tileId}|${tile.perCell ? `cell ${tile.cell.x},${tile.cell.y}` : `area ${tile.area}`}`;
 }
 
 /**