feat: shooter animation feel — directional lean, slide, composed airborne, weapon hold

Layer a procedural SkeletonModifier3D (ShooterPoseModifier) on top of the base
clip so the character reads like a movement-shooter avatar:

- Directional lean: banks into the movement direction (right/left/back) and
  blends smoothly for diagonals, driven by velocity relative to facing.
- Slide: leans the torso back and pitches the head up to look forward, instead
  of the base clip's forward-torso/legs-out "spine break".
- Airborne: plays a composed Jump pose (weapon ready) rather than a flailing
  fall.
- Weapon hold: the base clip already keeps the arms down (weapon at the hip);
  on ADS both arms lift and swing in toward centre-front to aim. ADS is read
  from the active weapon and synced (synced_is_ads) so remote players raise
  their weapons too.

The controller feeds movement direction + ADS to SkinnedPlayerModel.set_locomotion()
each frame (works for local and remote via synced velocity/rotation). All
rotations are authored in skeleton space (fwd=+Z, up=+Y, right=-X) and converted
per-bone; tuning constants are at the top of ShooterPoseModifier.

Verified by rendering the poses (idle/strafe/back/slide/ads) in a real Godot
viewport. Smoke test 30/30.

Co-Authored-By: Claude Fable 5 <[email protected]>

scenes/maps/procedural_arena/procedural_arena_runtime.gd

@@ -81,6 +81,7 @@ func _spawn_player(pid: int) -> CharacterBody3D:
 	client_rep_config.add_property(":synced_movement_state")
 	client_rep_config.add_property(":synced_movement_speed")
 	client_rep_config.add_property(":synced_is_crouching")
+	client_rep_config.add_property(":synced_is_ads")
 	client_rep_config.add_property(":synced_grapple_point")
 	client_rep_config.add_property(":synced_is_grapple_shooting")
 	client_rep_config.add_property(":synced_skin_id")