feat(machine): see-saw tipAxis option + a tuning lab course

createSeeSaw gains `tipAxis: 'x'|'z'` (default 'x' = unchanged). 'z' rocks the
plank fore↔aft so you can cross it in a Z-flowing course — the missing piece
that kept the see-saw out of every course (it was X-crossing only).

Adds ?course=seesaw, a throwaway lab for tuning the see-saw in isolation:
raised deck flush into the plank's near end (no ramp to misalign), teeter
across, drop off the far end onto the floor (a drop is never a wedge), fan for
momentum. Browser-verified it completes end-to-end.

Feel is still open (John's call): at crossing speed the plank barely pitches
(reads as a stable wobble-bridge). Knobs left exposed — maxTilt/restTilt on the
part, plank density/damping in the factory, fan force in the lab — to tune
"does it feel good" together. NOT wired into any shipped course.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/course/seesaw-lab.ts

@@ -0,0 +1,33 @@
+/**
+ * SEE-SAW LAB — a throwaway test course (?course=seesaw) for tuning the see-saw
+ * obstacle in isolation, away from the noise of a real level.
+ *
+ * Design that dodges the wedge problem: you roll along a RAISED DECK straight
+ * onto the plank's near end (both at y1.8 — a flush seam, no ramp to misalign),
+ * teeter across, and roll off the FAR end down onto the floor (a drop is never a
+ * wedge). A fan gives the momentum to carry over the pivot.
+ *
+ * Knobs to tune for feel (John's call): the see-saw's `maxTilt` (how much it
+ * rocks) and `restTilt` (start it level or pre-tipped), and the fan `force`.
+ */
+import type { CourseSpec } from './spec'
+
+export const SEESAW_LAB: CourseSpec = {
+  name: 'See-saw Lab',
+  spawn: [0, 2.3, 18],
+  finish: { xMin: -6, xMax: 6, zMin: -32, zMax: -20, yMax: 4 },
+  boxes: [
+    // base floor — catches the drop off the plank and carries the finish
+    { pos: [0, -0.5, -10], half: [10, 0.5, 32], color: '#efe3c6', roughness: 1 },
+    // raised deck you roll along into the plank's near end (top y1.8)
+    { pos: [0, 0.9, 8], half: [4, 0.9, 12], color: '#f7edd2', cast: true },
+    // finish pad on the floor, past the drop
+    { pos: [0, 0.4, -26], half: [6, 0.4, 4], color: '#FF6EB4', cast: true },
+  ],
+  machines: [
+    // fan on the deck — momentum to carry over the see-saw's pivot
+    { kind: 'fan', cfg: { id: 'sl-fan', position: [0, 2.2, 10], direction: [0, 0, -1], force: 30, range: 16, spread: 4 } },
+    // the see-saw: deck flush into the +Z end (z-4), drops off the -Z end (z-12) to the floor
+    { kind: 'seesaw', cfg: { id: 'sl-seesaw', position: [0, 1.6, -8], plankSize: [8, 0.4, 3.5], tipAxis: 'z', maxTilt: 0.35 } },
+  ],
+}

src/course/spec.ts

@@ -24,6 +24,7 @@ import type {
   SurfaceName,
 } from '../machine/index'
 import { TOASTER_ALLEY } from './toaster-alley'
+import { SEESAW_LAB } from './seesaw-lab'
 
 export interface FinishBox { xMin: number; xMax: number; zMin: number; zMax: number; yMax: number }
 
@@ -168,4 +169,5 @@ export function buildCourseFromSpec(
  *  game.ts and is the default). Load with `?course=<name>`. */
 export const COURSES: Record<string, CourseSpec> = {
   toaster: TOASTER_ALLEY,
+  seesaw: SEESAW_LAB,
 }

src/machine/parts.ts

@@ -262,19 +262,30 @@ export function createSpringBoot(world: World, cfg: SpringBootConfig): MachinePa
 export interface SeeSawConfig {
   id: string
   position: Vec3
-  /** Plank [length(x), thickness(y), width(z)]. Default [8, 0.4, 2.4]. */
+  /** Plank [length, thickness, width]. Length runs along the crossing axis
+   *  (see tipAxis); width is the other horizontal. Default [8, 0.4, 2.4]. */
   plankSize?: Vec3
+  /** Which horizontal axis you CROSS along: 'x' (default, tips left↔right) or
+   *  'z' (tips fore↔aft — for a course that runs along Z). */
+  tipAxis?: 'x' | 'z'
   /** Max tilt each way, radians. Default 0.5 (~29°). */
   maxTilt?: number
-  /** Initial tilt (rad, about Z). Negative = +X end starts low. Default 0 (level). */
+  /** Initial tilt (rad). Sign follows the tip axis; 0 = level. Default 0. */
   restTilt?: number
 }
 
 export function createSeeSaw(world: World, cfg: SeeSawConfig): MachinePart {
   const { physics, rapier, scene } = world
   const pos = asVec3(cfg.position)
-  const [px, py, pz] = cfg.plankSize ?? [8, 0.4, 2.4]
+  const [len, py, wid] = cfg.plankSize ?? [8, 0.4, 2.4]
   const maxTilt = cfg.maxTilt ?? 0.5
+  // Crossing axis → plank footprint + the axis it rocks about. 'z' runs the long
+  // side along Z and tips about X, so you roll across it in a Z-flowing course.
+  const tipZ = cfg.tipAxis === 'z'
+  const hx = (tipZ ? wid : len) * 0.5
+  const hz = (tipZ ? len : wid) * 0.5
+  const tiltAxis = tipZ ? new THREE.Vector3(1, 0, 0) : new THREE.Vector3(0, 0, 1)
+  const jointAxis = tipZ ? { x: 1, y: 0, z: 0 } : { x: 0, y: 0, z: 1 }
 
   const group = new THREE.Group()
   scene.add(group)
@@ -295,7 +306,7 @@ export function createSeeSaw(world: World, cfg: SeeSawConfig): MachinePart {
 
   // dynamic plank (optionally pre-tilted so delivery direction is deterministic)
   const restTilt = cfg.restTilt ?? 0
-  const q0 = new THREE.Quaternion().setFromAxisAngle(new THREE.Vector3(0, 0, 1), restTilt)
+  const q0 = new THREE.Quaternion().setFromAxisAngle(tiltAxis, restTilt)
   const plankBody = physics.createRigidBody(
     rapier.RigidBodyDesc.dynamic()
       .setTranslation(pos.x, pos.y, pos.z)
@@ -303,22 +314,22 @@ export function createSeeSaw(world: World, cfg: SeeSawConfig): MachinePart {
       .setAngularDamping(0.6),
   )
   physics.createCollider(
-    rapier.ColliderDesc.cuboid(px * 0.5, py * 0.5, pz * 0.5)
+    rapier.ColliderDesc.cuboid(hx, py * 0.5, hz)
       .setDensity(0.4)
       .setFriction(1.0),
     plankBody,
   )
 
-  // revolute joint about Z → the plank tips in the X/Y plane (blob rolls along X)
+  // revolute joint about the tip axis → the plank rocks so you roll across it
   const jd = rapier.JointData.revolute(
-    { x: 0, y: 0, z: 0 }, { x: 0, y: 0, z: 0 }, { x: 0, y: 0, z: 1 },
+    { x: 0, y: 0, z: 0 }, { x: 0, y: 0, z: 0 }, jointAxis,
   )
   jd.limitsEnabled = true
   jd.limits = [-maxTilt, maxTilt]
   physics.createImpulseJoint(jd, anchor, plankBody, true)
 
   const plank = new THREE.Mesh(
-    new THREE.BoxGeometry(px, py, pz),
+    new THREE.BoxGeometry(hx * 2, py, hz * 2),
     new THREE.MeshStandardMaterial({ color: '#a1887f', roughness: 0.7 }),
   )
   plank.castShadow = true