#124 bench: the ZONE-WALK MATRIX -- systematic per-panel targeting, operator-watchable

The precision rig the operator specified: two nodes, target visible,
every zone aimed at deliberately through the REAL reticle/pick path.

* BT_SPIN_SELF=<deg/s> (target node): rotates the node's own viewpoint
  mech in place (the BT_SPAWN_AT write pattern per frame) so every
  aspect passes the shooter's boresight; the operator watches this
  node's paper doll take the hits.
* BT_ZONE_WALK=<secs/zone> (shooter node): walks the target's damage
  zones in order, resolves each zone's carrier segment's live world
  position (BTResolveSegmentWorld + the new BTMechZoneSegAndName
  bridge), SERVOS the torso twist + aim elevation until the centered
  reticle ray (BTGetAimRay) points at the segment, fires 3 laser
  pulses, advances. [walk] ZONE/FIRE/HOLD on A pairs with [dmghit]
  zone/level lines on B.
* scratchpad/night10/zonewalk.sh: launch both + relay, map sed'd to
  grass/day, NO kill timer -- the session stays up for observation.

Hard-won servo constraints (documented in test-harness.md so they are
never relearned): the ENGAGE GATE (ray live + range<150 + bearing<1.1;
outside it RELAX the twist -- servoing at the twist limit while the
goto marches is a limit-clamp fight that visibly shakes the mech, and
a 0.55 gate deadlocks against the goto's ~0.55 resting bearing); YAW
POLARITY -1 (the twist cell's angular sense is opposite atan2(x,-z)
world yaw -- operator-observed live, the SECOND witness for the #124
SelectSlice twist-sign flip) with a divergence watchdog that
self-flips; damped correction (gain .40, cap .025/frame -- the aim ray
lags the twist write a frame); and a one-shot approach PORT to 100u
off the target instead of a cross-map march.

First live run: walker cycles zones, settles, fires; the target's
luleg climbed 0.458->0.523 under its own aimed pulses -- aimed shot,
correct panel, damage consumed.

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

context/test-harness.md

@@ -90,6 +90,43 @@ own1 snd1 thr1 vul1 (own1 has NO arm zones — rack zones instead).
 against cavern rock and they shoot terrain for the whole window. Always sed the
 map/time on the COPY, single-node and two-node alike.
 
+### THE ZONE-WALK MATRIX (`scratchpad/night10/zonewalk.sh`) — systematic
+### per-panel hit-location testing, operator-watchable (built for #124)
+The precision-targeting rig: **two nodes**. Node B (launched first, back
+window) is the TARGET — `BT_SPIN_SELF=15` rotates its own viewpoint mech in
+place (deg/s; the BT_SPAWN_AT write pattern, applied per frame) so every
+aspect passes the shooter's boresight; its window shows its own paper doll
+taking the hits (`BT_DMG_LOG=1` prints every `[dmghit]` with zone + level).
+Node A (foreground — it owns the pick focus) runs `BT_ZONE_WALK=<secs/zone>`:
+walks the target's damage zones in index order, resolves each zone's CARRIER
+SEGMENT's live world position (`BTResolveSegmentWorld` — the same resolver the
+attached smoke rides; zone→segment via the `BTMechZoneSegAndName` bridge),
+SERVOS the torso twist + aim elevation until the centered reticle's pick ray
+(`BTGetAimRay(0,0)`) points at the segment, then pulses the laser trigger 3×
+and advances. `[walk] ZONE/FIRE/HOLD` lines on A pair with `[dmghit]` on B.
+Launch: `bash scratchpad/night10/zonewalk.sh` (relay included; NO kill timer —
+the session stays up for observation; teardown = taskkill btl4).
+Companion scalpels: `BT_ASPECT_TEST=1` (the zero-premise frame probe — 4
+world-cardinal self-impacts + one at each weapon's physical MUZZLE, the
+left/right anchors that caught the #124 reflection-vs-rotation error);
+`BT_AIM_SWEEP=<period>` (blind elevation sweep — superseded by the walker).
+Hard-won constraints baked into the walker (do not relearn these):
+- **Engage gate**: servo only when the aim ray is live AND range < 150 AND
+  |bearing| < 1.1 rad; otherwise RELAX the twist toward 0 and hold the zone
+  clock. Without it the servo winds the twist to its limit while the goto
+  marches (limit-clamp fight = the mech visibly SHAKES) — and the gate must
+  sit well above the goto's resting bearing error (~0.55 rad at STOP=90) or
+  it deadlocks in HOLD.
+- **Yaw polarity**: the twist cell's angular sense is OPPOSITE atan2(x,−z)
+  world yaw (operator-observed live: `+=` turned the torso AWAY — the second
+  witness for the #124 SelectSlice twist-sign flip). The servo starts at −1
+  and carries a growth WATCHDOG that self-flips if the error diverges.
+- **Damped servo**: gain 0.40, step cap 0.025 rad/frame, deadband — the aim
+  ray lags the twist write by a frame; full-error correction oscillates.
+- **Approach port**: dropzones spawn cross-map; the walker one-shot teleports
+  A to 100 u off the target (BT_SPAWN_AT pattern incl. the +500 y ground-snap
+  lift) instead of minutes of marching.
+
 ### Two-node bench skeleton (the MP pattern)
 ```bash
 bt_assert_player_env