feat(animation): decompose AnimationController into FSM-based architecture

Replace the 2,200-line monolithic AnimationController (goto-driven,
single class, untestable) with a composed FSM architecture per
refactor.md.

New subsystem (src/rendering/animation/ — 16 headers, 10 sources):
- CharacterAnimator: FSM composer implementing ICharacterAnimator
- LocomotionFSM: idle/walk/run/sprint/jump/swim/strafe
- CombatFSM: melee/ranged/spell cast/stun/hit reaction/charge
- ActivityFSM: emote/loot/sit-down/sitting/sit-up
- MountFSM: idle/run/flight/taxi/fidget/rear-up (per-instance RNG)
- AnimCapabilitySet + AnimCapabilityProbe: probe once at model load,
  eliminate per-frame hasAnimation() linear search
- AnimationManager: registry of CharacterAnimator by GUID
- EmoteRegistry: DBC-backed emote command → animId singleton
- FootstepDriver, SfxStateDriver: extracted from AnimationController

animation_ids.hpp/.cpp moved to animation/ subdirectory (452 named
constants); all include paths updated.

AnimationController retained as thin adapter (~400 LOC): collects
FrameInput, delegates to CharacterAnimator, applies AnimOutput.

Priority order: Mount > Stun > HitReaction > Spell > Charge >
Melee/Ranged > CombatIdle > Emote > Loot > Sit > Locomotion.
STAY_IN_STATE policy when all FSMs return valid=false.

Bugs fixed:
- Remove static mt19937 in mount fidget (shared state across all
  mounted units) — replaced with per-instance seeded RNG
- Remove goto from mounted animation branch (skipped init)
- Remove per-frame hasAnimation() calls (now one probe at load)
- Fix VK_INDEX_TYPE_UINT16 → UINT32 in shadow pass

Tests (4 new suites, all ASAN+UBSan clean):
- test_locomotion_fsm: 167 assertions
- test_combat_fsm: 125 cases
- test_activity_fsm: 112 cases
- test_anim_capability: 56 cases

docs/ANIMATION_SYSTEM.md added (architecture reference).

include/rendering/camera_controller.hpp

@@ -98,6 +98,10 @@ public:
     using StandUpCallback = std::function<void()>;
     void setStandUpCallback(StandUpCallback cb) { standUpCallback_ = std::move(cb); }
 
+    // Callback invoked when the player sits down via local input (X key).
+    using SitDownCallback = std::function<void()>;
+    void setSitDownCallback(SitDownCallback cb) { sitDownCallback_ = std::move(cb); }
+
     // Callback invoked when auto-follow is cancelled by user movement input.
     using AutoFollowCancelCallback = std::function<void()>;
     void setAutoFollowCancelCallback(AutoFollowCancelCallback cb) { autoFollowCancelCallback_ = std::move(cb); }
@@ -310,6 +314,7 @@ private:
     // Movement callback
     MovementCallback movementCallback;
     StandUpCallback standUpCallback_;
+    SitDownCallback sitDownCallback_;
     AutoFollowCancelCallback autoFollowCancelCallback_;
 
     // Movement speeds