Colliders take a frame's outline and tiles have their own surface (#193)

Issue #193 asked where friction, bounciness and density of a tile
live. Tiles had a fixed surface (friction 0.5, bounciness 0) and a
collider could not use the outline drawn on a texture frame (#141).
Both gaps are closed here.

- A collider has a fifth shape "Frame collision" with a Frame row.
  The frame is picked like a sprite's texture and stored as the
  value 'frame'.
- The shape is linked, not copied. Redrawing the outline in the
  texture tab changes every collider using that frame, in the editor
  and on the next play.
- The frame centre sits on the collider origin, one image pixel is
  one unit. The shape turns, scales and mirrors like any other.
- Every outline ring blocks. Each ring is split into convex parts the
  same way a polygon is.
- Without a frame, with a whole texture or with a frame that has no
  outline, the collider is drawn red as a box and blocks nothing.
- Move, Rapier, raycast, detection, hit test, drawing and footprint
  all resolve the frame. At runtime an entity looks the frame up
  live in the current project.
- A tile has Friction and Bounciness next to Solid and One-way. The
  rows only show when Solid is on. Defaults and mixing rules match
  the collider. Density is left out because a tilemap never moves.
- Tilemap cells are grouped by surface as well as by one-way angle.
  Ice and stone never merge into one rectangle, and every cell brings
  its surface to Move and Rapier.

Findings:

- A tile with a collision scale below 1 is no longer flush with a
  neighbour of another surface. It is inset there like at an open
  side, the same as next to one-way tiles today.
- A bounce test first measured 0.84. The ball had landed on the hero
  at the origin, not on the tile. On the tile it measures 0.993.

Tests:

- ./check.sh: 130 files, 1931 tests, exit 0 (was 129 / 1917).
- 14 new unit tests in tests/core/surface.test.ts. The Move tests run
  against the editor run and the physics system.
- 3 new e2e checks: 40l (surface rows follow Solid), 42e (collider
  has frame size, a walker climbs the slope in the frame).
- GM_STEPS=1-8,39-43 freshly seeded: ALL PASSED, 376 checks.
- Three counter-probes bite: Move ignores the tile surface (2 fail),
  Rapier ignores it (1 fails), frame shape falls back to a box
  (5 fail).
- Check sum 1198 is computed (1195 + 3), the full run belongs to CI.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/runtime/physics/PhysicsSystem.ts

@@ -12,6 +12,7 @@ import type { Vec2 } from '../../core/model/Properties';
 import { signedArea } from '../../core/model/polygon';
 import { parentChain } from '../../core/model/walk';
 import { collisionFilterOf, interactionGroups } from '../../core/model/collisionLayers';
+import type { TileMaterial } from '../../core/model/tilemapCollision';
 import {
   type ColliderShape,
   contactBetween,
@@ -47,6 +48,7 @@ import {
   shapeOf,
   type SolidShape,
   Solids,
+  surfaceOf,
 } from '../systems/solids';
 import type { System } from '../systems/System';
 import { type Rapier, toMetres, toPixels } from './engine';
@@ -507,10 +509,11 @@ export class PhysicsSystem implements System, CollisionSource {
     const local = shapes.map((solid) => ({ ...solid, pieces: piecesInBody(origin, solid.shape) }));
     const shapeKey =
       JSON.stringify(
-        local.map(({ entity, pieces, facing }) => [
+        local.map(({ entity, pieces, facing, surface }) => [
           entity.id,
           pieces.map(pieceKey),
           facing && [round(facing.x), round(facing.y), facing.cone],
+          surface ?? 0,
           entity.type === 'collider' ? [isTrigger(entity.props), colliderMaterial(entity.props)] : 0,
           // The layers sit on the engine collider, so changing one has to rebuild it (#152).
           interactionGroups(collisionFilterOf(entity.props)),
@@ -530,10 +533,10 @@ export class PhysicsSystem implements System, CollisionSource {
         (sum, { entity, shape }) => sum + (isSolid(entity) ? shapeArea(shape) : 0),
         0,
       );
-      for (const { entity, shape, facing, pieces } of local)
+      for (const { entity, shape, facing, surface, pieces } of local)
         for (const piece of pieces) {
           const share = mass > 0 && area > 0 ? (mass * pieceArea(piece)) / area : 0;
-          const desc = this.colliderDesc(entity, piece, share, facing !== null);
+          const desc = this.colliderDesc(entity, piece, share, facing !== null, surface);
           if (!desc) continue;
           const collider = this.engine!.createCollider(desc, body.body);
           body.colliders.push(collider);
@@ -574,7 +577,13 @@ export class PhysicsSystem implements System, CollisionSource {
   }
 
   /** The engine collider of one piece; null for a piece of no size, which the engine would refuse. */
-  private colliderDesc(entity: Entity, piece: Piece, mass: number, oneWay: boolean): ColliderDesc | null {
+  private colliderDesc(
+    entity: Entity,
+    piece: Piece,
+    mass: number,
+    oneWay: boolean,
+    surface?: TileMaterial,
+  ): ColliderDesc | null {
     const R = this.rapier;
     if (pieceArea(piece) <= 0) return null;
     let desc: ColliderDesc | null;
@@ -597,9 +606,10 @@ export class PhysicsSystem implements System, CollisionSource {
     if (!desc) return null;
     if (piece.kind !== 'convex')
       desc.setTranslation(toMetres(piece.x), toMetres(piece.y)).setRotation(piece.angle);
-    // A tile has no material of its own; it gets what a new collider starts with.
+    // A tile's cell brings its tile's surface (#193); a density it does not need, a map never moves.
     const material = colliderMaterial(entity.type === 'collider' ? entity.props : {});
-    desc.setFriction(material.friction).setRestitution(material.bounciness);
+    const { friction, bounciness } = surfaceOf(entity, surface);
+    desc.setFriction(friction).setRestitution(bounciness);
     // The livelier side wins, as in Box2D: a ball that should bounce does so on plain ground too.
     desc.setRestitutionCombineRule(R.CoefficientCombineRule.Max);
     if (!isSolid(entity)) desc.setSensor(true).setDensity(0);