Add scene-authored FX effect library and Fx autoload

Replace code-buried CPUParticles with inspector-editable effect scenes fired
through one Fx autoload: code names the effect (Fx.spawn(&"blood_burst", xform)),
the .tscn owns the look. Ships four placeholder effects (blood_burst, slam_dust,
spark, hit_flash) seeded from the old code params, a self-freeing one_shot_effect
root helper, a live preview gallery, and an FX style guide for Siim. CPUParticles
only (mobile GLES3 constraint). Existing call sites untouched -- migrated later.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

project.godot

@@ -29,6 +29,7 @@ Settings="*res://settings.gd"
 Run="*res://levels/run_state.gd"
 OrientationGuard="*res://orientation_guard.gd"
 WebStartGate="*res://web_start_gate.gd"
+Fx="*res://fx/fx.gd"
 
 [display]