Move bull ability FX into the authorable systems/fx/ library

The bull's kick/dash/slam/roll and movement particles were ~112 lines of
hand-built CPUParticles3D in player.gd, bypassing the Fx system entirely
(Fx.spawn had zero callers) — so Siim couldn't author them in the inspector.

- Add six inspector-editable effect scenes (bull_slam/kick/dash one-shots +
  bull_hoof_dust/charge_trail/roll_sparks continuous); wire player.gd to
  Fx.spawn() / new Fx.attach(); register them + the gallery + smoke test.
- Extract the coded, non-particle slam rings + kick flash into a new
  bull_impact_fx.gd component (bull_legs.gd pattern).
- Drop 12 now-obsolete DP dust look-params (the .tscn owns the look now).
- Trim god-file comment noise: rewrite an AI-placeholder barrel block to typed
  code, drop dead commented-out bounce/topple code, fix stale post-reorg file
  paths (Assets/… → art/…, PascalCase scene names → snake_case).
- Document the Siim workflow in systems/fx/README.md; update CLAUDE.md.

player.gd 1307 → 944 lines. Full suite green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CCyaJPTHCH9EDthpf4A5Sj

tests/bear_fx_test.gd

@@ -1,6 +1,6 @@
 extends SceneTree
 ## Bear attack-FX test. The four attack effects (crater, smash debris, left/right claw slash)
-## live in Assets/fx/Bear_FX.glb as meshes that rest at scale 0 at Siim's baked positions. The
+## live in art/fx/bear_fx.glb as meshes that rest at scale 0 at Siim's baked positions. The
 ## GLB carries its own AnimationPlayer whose clips scale one mesh up at its strike frame and
 ## back to 0 (self-hiding) while pinning the others at 0. bear.gd fires the matching clip on
 ## each strike (see _play_fx). This asserts each clip pops its own mesh, self-hides by the end,