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/Physics/Bot_LowFriction.physicsMaterial

@@ -0,0 +1,15 @@
+%YAML 1.1
+%TAG !u! tag:unity3d.com,2011:
+--- !u!134 &13400000
+PhysicsMaterial:
+  m_ObjectHideFlags: 0
+  m_CorrespondingSourceObject: {fileID: 0}
+  m_PrefabInstance: {fileID: 0}
+  m_PrefabAsset: {fileID: 0}
+  m_Name: Bot_LowFriction
+  serializedVersion: 2
+  m_DynamicFriction: 0.3
+  m_StaticFriction: 0.3
+  m_Bounciness: 0
+  m_FrictionCombine: 2
+  m_BounceCombine: 2