bot: fix slow/unsteerable driving + controls tooltip

Diagnosis (agent + live values): tuning & tread geometry were fine, debugAutoDrive
off — the real drag was the box chassis sliding on default ~0.6 friction, which ate
forward speed AND resisted yaw (dead steering).
- Bot_LowFriction physics material (0.3, Minimum combine) on the chassis → slides +
  yaws freely.
- TreadPart yaw-assist (220 N·m, sign reinforces natural differential) so a throttle
  difference gives a clear turn; cancels to zero on equal throttle (straight kept).
- SeatManager: context-aware controls tooltip under the seat box (on-foot vs driving),
  with a 'turn: differ vs teammate' hint.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Assets/Scripts/Bot/TreadPart.cs

@@ -63,6 +63,9 @@ namespace Jankenbots.Prototype
         [Range(0f, 0.5f)]
         public float throttleDeadzone = 0.08f;
 
+        [Tooltip("Extra yaw torque (N·m) at full throttle, REINFORCING the natural differential turn so steering reads clearly even with ground friction. Left/Right push opposite ways, so equal throttle cancels to zero → straight line preserved. Set 0 to rely on pure emergent differential. Flip sign in code if it ever fights the turn.")]
+        public float yawAssistTorque = 220f;
+
         // ---- (b) LOCK-PIVOT tuning -----------------------------------------------
         [Header("(b) Lock-pivot — plant this tread as an anchor")]
         [Tooltip("How hard the planted tread resists the chassis sliding at its position.")]
@@ -192,6 +195,13 @@ namespace Jankenbots.Prototype
             {
                 Vector3 drive = transform.forward * (_throttle * cruiseForce);
                 chassis.AddForceAtPosition(drive, treadPos, ForceMode.Force);
+
+                // Yaw assist: reinforce the differential turn (Left throttles → nose right,
+                // Right throttles → nose left). Cancels to zero when both throttle equally,
+                // so straight-line driving is untouched; the maxAngularVelocity clamp still
+                // caps total spin so this can't reintroduce an uncontrolled tip.
+                float yawSign = side == Side.Left ? 1f : -1f;
+                chassis.AddTorque(chassis.transform.up * (yawSign * _throttle * yawAssistTorque), ForceMode.Force);
             }
 
             // ---- (c) PASSIVE LEAN — anti-tip ballast ---------------------------