Lane D R17 (v3.2): the drummer's pelvic-lean (ledger #1)

The R16 seated drummer sat bolt-upright (the sit clip can't carry a pelvic tilt
— Hips.quaternion is unusable on these rigs). Fix: a small constant forward torso
lean applied POST-MIX. spawnRig({seated}) caches the mid-spine bone (seatBone);
seatedLean(spine) rotates it forward ~0.34 rad each frame after the mixer resets
it (constant offset, not accumulation); band.js calls it for m.seated members.
The drummer leans into the kit — head 0.28m forward of the hips, reads as a real
drummer from the side.

Byte-identical (my signature): standing rigs get no seatBone (found only in the
seated block) and seatedLean runs for seated figs only. Verified with the lean
live — walkers 188/188 face travel, keeper standing rig (stature 1.718,
hasSeatBone:false, innerRotY pi). No existing consumer moves.

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

docs/LANES/LANE_D_NOTES.md

@@ -6,6 +6,25 @@ _rotOnly/head-bone normalize/upgradeStreetPeople). Measurements on the M3 Ultra
 
 ---
 
+## ROUND 17 — v3.2 THE TAIL: the drummer's pelvic-lean (ledger #1)
+
+*PROCITY-D, 2026-07-16. The R16 honest limit — the upright bent-leg sit read stiff — is fixed. Files
+`rigs.js`, `band.js`. Byte-identical proof holds (my signature discipline).*
+
+The sit clip can't carry a pelvic tilt (`Hips.quaternion` is unusable on these rigs — R16), so the R16
+drummer sat bolt-upright. **Fix: a small constant forward torso lean applied POST-MIX.** `rigs.js`
+`spawnRig({seated})` now caches the mid-spine bone (`seatBone`), and `seatedLean(spine)` rotates it forward
+~0.34 rad each frame AFTER the mixer resets it (so it's a constant offset, never an accumulation). `band.js`
+calls it for `m.seated` members only. The drummer now leans into the kit — verified **head 0.28 m forward
+of the hips**, reads as a real drummer from the side (the R16 stiffness is gone).
+
+**Byte-identical proof**: standing rigs get **no `seatBone`** (found only in `spawnRig`'s seated block) and
+`seatedLean` is only called for seated figs. Verified with the lean live: streamed walkers **188/188** face
+travel, keeper is a standing rig (stature **1.718**, `hasSeatBone:false`, `innerRotY:π`). No existing
+consumer moves. Seated-only, parallel path — same discipline as R16.
+
+---
+
 ## ROUND 16 — v3.1 THE FLIP: the drummer finally sits + backline unification
 
 *PROCITY-D, 2026-07-16. Ledger #2 (sit-clip wiring) + #5 (amp at C's `backline[]`). Files `rigs.js`,

web/js/citizens/band.js

@@ -16,7 +16,7 @@
 
 import * as THREE from 'three';
 import { rng, shuffle } from '../core/prng.js';
-import { pickRig, spawnRig } from './rigs.js';
+import { pickRig, spawnRig, seatedLean } from './rigs.js';
 import { makePlaceholder } from './placeholder.js';
 import { loadGLB } from '../core/loaders.js';
 
@@ -271,6 +271,7 @@ export class GigCrew {
     for (const m of this.members) {
       if (m.actor.mixer) m.actor.mixer.update(dt);      // rig idle
       else m.actor.tick && m.actor.tick(dt, false);     // placeholder idle
+      if (m.seated) seatedLean(m.actor.seatBone);       // R17: tilt the drummer forward into the kit (post-mix)
       const f = m.actor.fig, b = m.base, ph = m.phase;
       if (m.part === 'band') {
         if (m.seated) {                                  // drummer: seated bob + a little shoulder work, planted

web/js/citizens/rigs.js

@@ -189,6 +189,7 @@ export function spawnRig(rig, { ry = 0, clip = null, height = 1.75, phase = 0, s
   const mixer = new THREE.AnimationMixer(inner);
   const chosen = clip || rig.anims.find(c => /idle/i.test(c.name)) || rig.anims[1] || rig.anims[0];
   const act = _action(mixer, inner, chosen);
+  let seatBone = null;
   if (act) {
     act.time = phase * (act.getClip().duration || 0); act.play();
     // R16 SEATED re-plant (OPT-IN — standing spawns skip this, so keepers/browsers/band are byte-identical).
@@ -202,10 +203,22 @@ export function spawnRig(rig, { ry = 0, clip = null, height = 1.75, phase = 0, s
       let m = Infinity;
       inner.traverse(o => { if (o.isBone) { o.getWorldPosition(_wp); m = Math.min(m, _wp.y); } });
       if (isFinite(m)) inner.position.y -= m;
+      // R17 pelvic-lean: cache the mid-spine bone so seatedLean() can tilt the torso forward each frame (the
+      // sit clip can't — Hips.quaternion is unusable on these rigs). Seated-only; standing rigs get no seatBone.
+      inner.traverse(o => { if (!seatBone && o.isBone && /Spine1$/i.test(o.name)) seatBone = o; });
     }
   }
   function dispose() { mixer.stopAllAction(); mixer.uncacheRoot(inner); _disposeInner(inner); }
-  return { fig, inner, mixer, height, head, dispose };
+  return { fig, inner, mixer, height, head, seatBone, dispose };
+}
+
+// R17 pelvic-lean: tilt a seated fig's torso forward, applied AFTER the mixer each frame. The mixer resets
+// the bone's quaternion from the clip every frame, so this is a CONSTANT offset (not an accumulation). Only
+// seated figs (drummer, bench-sit loiterers) call it; standing rigs have no seatBone → never leaned.
+const _LEAN_AXIS = new THREE.Vector3(1, 0, 0);
+const _LEAN_Q = new THREE.Quaternion();
+export function seatedLean(spine, angle = 0.34) {
+  if (spine) spine.quaternion.multiply(_LEAN_Q.setFromAxisAngle(_LEAN_AXIS, angle));
 }
 
 // ---- makeActor: near-tier pedestrian with walk↔idle crossfade (built at nominal height; the sim