Body rebuild step 0: Generic6DOFJoint3D on Box3D (fork patch 2) — ball w/ cone/twist + torque-capped muscle motor, weld, drive

- addons/godot-box3d/bin/libgodot-box3d.so rebuilt from fork branch generic-6dof-joint
  (weld patch + 6DOF); .vendor/box3d-generic-6dof.patch exported
- SixDofSpike: type, ball pivot/swing, cone+twist limits, muscle holds 2.94 N·m lever at 10 N·m
  cap and fails at 1.5, tracks moving target, drive follows + push-off reaction, weld rigid,
  live flag toggles — PASS x3; all existing spikes PASS x3 on the new binary
- research/godot-box3d-6dof.md (mapping, units, C# muscle recipe); HANDOFF/AGENTS pointers

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scenes/sixdof_spike.tscn

@@ -0,0 +1,6 @@
+[gd_scene load_steps=2 format=3]
+
+[ext_resource type="Script" path="res://scripts/SixDofSpike.cs" id="1_spike"]
+
+[node name="SixDofSpike" type="Node3D"]
+script = ExtResource("1_spike")