Add "lift kit" for Campaign 2 bus

Campaign 2's larger wheels now hang further under the chassis via a new `wheelRestOffsetY` theme knob (42 vs 34 design units), making the bus visibly sit taller. The wishbone anchor and initial wheel position derive from this per-campaign offset, so both size and mount point stay correctly centered on the axle line.

- Add `BUS.wheelRestOffsetY2` in config
- Expose `wheelRestOffsetY` in theme `bus` blocks (default = campaign 1)
- Use themed offset in Bus constructor for anchor and wheel placement
- Update docs/comments to explain the lift-kit behavior

sprites.md

@@ -46,10 +46,13 @@ with it (same ratios, just re-derive from that one constant).
   speed than fine detail.
 - Campaign 2 uses its own wheel image, `bus_wheel_2.png` (bigger - it's
   displayed at a ~1.2x radius around the same axle center, selected via the
-  `bus` block in `src/data/themes.js`), and its own chassis image,
+  `bus` block in `src/data/themes.js`), its own chassis image,
   `bus_chassis_2.png` (same 340x128 design box, physics body unchanged -
-  only the texture is swapped per campaign).
+  only the texture is swapped per campaign), and a "lift kit": the wheel
-  Both wheels still share one image per campaign.
+  mount point sits ~8 design units lower under the chassis (see
+  `wheelRestOffsetY2` in `src/config.js` and the `bus` block in
+  `src/data/themes.js`), so it reads as a lifted bus. Both wheels still
+  share one image per campaign.
 - If you want a bus with different proportions (longer, taller, etc.) than
   340x128, tell me the size you want art at - that number is also the physics
   body size in `src/config.js` (`BUS.chassisWidth`/`chassisHeight`), so it

src/config.js

@@ -41,6 +41,10 @@ export const BUS = {
   // bus_chassis.png's painted wheel wells (a gap opens above the tire),
   // a deliberate look/feel tradeoff rather than a measurement.
   wheelRestOffsetY: 34 * WORLD_SCALE,
+  // Campaign 2's "lift kit": the same mount point 8 design units lower
+  // (see `bus` in data/themes.js), so the bigger wheels hang further under
+  // the chassis and the bus visibly sits taller.
+  wheelRestOffsetY2: 42 * WORLD_SCALE,
   // Horizontal spread between the two suspension links each wheel hangs
   // from (a "wishbone") - a single point-to-point constraint can't resist
   // sideways drift at all (it's radially symmetric), so each wheel is held

src/data/themes.js

@@ -82,13 +82,16 @@ export const DEFAULT_THEME = {
   // `chassisKey` is the chassis texture (purely visual - the physics body
   // stays the same BUS.chassisWidth/Height rectangle either way);
   // `wheelTextureKey` is the texture both wheels render with; `wheelRadius`
-  // is the physics/display radius in world pixels. Changing the radius
+  // is the physics/display radius in world pixels; `wheelRestOffsetY` is
-  // keeps the wheel centered on the same spot (the wishbone anchors and
+  // the wheel mount point, local to the chassis (y down) - raising it is
-  // wheelRestOffsetY are radius-agnostic) - only the size changes.
+  // the "lift kit" knob (campaign 2). A bigger radius keeps the wheel
+  // centered on its axle (the wishbone anchors derive from these
+  // offsets, so they stay put) - only the size changes.
   bus: {
     chassisKey: 'bus_chassis',
     wheelTextureKey: 'bus_wheel',
     wheelRadius: BUS.wheelRadius,
+    wheelRestOffsetY: BUS.wheelRestOffsetY,
   },
   // The three parallax layers, back to front. `far` is the full-viewport
   // sky; `mid`/`near` are bottom-anchored ground tiles (see PlayScene's
@@ -122,6 +125,10 @@ export const THEMES = {
       chassisKey: 'bus_chassis_2',
       wheelTextureKey: 'bus_wheel_2',
       wheelRadius: BUS.wheelRadius2,
+      // "Lift kit": drop the wheel mount point 8 design units below
+      // campaign 1's (see BUS.wheelRestOffsetY2) - the bus sits taller and
+      // the bigger wheels hang lower under the body.
+      wheelRestOffsetY: BUS.wheelRestOffsetY2,
     },
   },
   // Add campaign03: { ... } here when it's ready.

src/entities/Bus.js

@@ -11,20 +11,26 @@ export default class Bus {
   constructor(scene, x, y, angle = 0, busTheme = null) {
     this.scene = scene;
     const matter = scene.matter;
-    // Per-campaign bus look (see `bus` in data/themes.js): which chassis
+    // Per-campaign bus look (see `bus` in data/themes.js): chassis and
-    // texture and wheel texture to render and the physics/display wheel
+    // wheel texture, the physics/display wheel radius, and the wheel
-    // radius. Defaults to campaign 1's values, so existing callers (and
+    // mount point (wheelRestOffsetY - campaign 2's "lift kit", which drops
-    // any bus built outside a themed scene) are unchanged. The chassis
+    // the wheels further under the chassis). Defaults to campaign 1's
-    // texture is visual only - the physics body stays the same
+    // values, so existing callers (and any bus built outside a themed
-    // BUS.chassisWidth/Height rectangle either way. NOTE: only the wheel
+    // scene) are unchanged. The chassis texture is visual only - the
-    // size varies - the wheel's CENTER is anchored to the chassis via the
+    // physics body stays the same BUS.chassisWidth/Height rectangle
-    // wishbone below (anchorY/wheelRestOffsetY are radius-agnostic), so a
+    // either way. (The wishbone anchors below are DERIVED from these
-    // bigger radius grows the wheel around the same center, exactly as
+    // values, so a bigger radius and/or a lower mount point both stay
-    // intended.
+    // correctly centered on the same axle line.)
-    const { chassisKey = 'bus_chassis', wheelTextureKey = 'bus_wheel', wheelRadius = BUS.wheelRadius } = busTheme || {};
+    const {
+      chassisKey = 'bus_chassis',
+      wheelTextureKey = 'bus_wheel',
+      wheelRadius = BUS.wheelRadius,
+      wheelRestOffsetY = BUS.wheelRestOffsetY,
+    } = busTheme || {};
     this.chassisKey = chassisKey;
     this.wheelTextureKey = wheelTextureKey;
     this.wheelRadius = wheelRadius;
+    this._wheelRestOffsetY = wheelRestOffsetY;
 
     this.group = matter.world.nextGroup(true);
     // Dedicated category so the compartment fixtures below can target kids
@@ -50,7 +56,10 @@ export default class Bus {
     // so the spring pulling the wheel out to its full constraint length is
     // what lands it exactly on wheelRestOffsetY (anchorY + travel = restY),
     // while still leaving suspensionTravel of compress/droop room.
-    const anchorY = BUS.wheelRestOffsetY - BUS.suspensionTravel;
+    // this._wheelRestOffsetY is the campaign's mount point (the lift-kit
+    // knob) - raising it lowers the wheel under the chassis and, since
+    // the anchor rides with it, the bus visibly sits taller.
+    const anchorY = this._wheelRestOffsetY - BUS.suspensionTravel;
     // The wheel doesn't hang straight down from a single point (that point
     // is split into two, spread by axleSpread), so the constraint's target
     // length is the hypotenuse to the spread anchor, not just the vertical
@@ -74,7 +83,7 @@ export default class Bus {
     // alpha.
     this.chassis.setDepth(2);
 
-    const wheelY = y + BUS.wheelRestOffsetY;
+    const wheelY = y + this._wheelRestOffsetY;
     this.wheelRear = this._createWheel(x + BUS.rearWheelOffsetX, wheelY, this.wheelTextureKey, this.wheelRadius);
     this.wheelFront = this._createWheel(x + BUS.frontWheelOffsetX, wheelY, this.wheelTextureKey, this.wheelRadius);