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>

fx/README.md

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+# FX — the effect library
+
+Gameplay code fires a **named** effect; the **scene** decides how it looks. That split is the whole
+point: a programmer writes `Fx.spawn(&"blood_burst", hit_transform)` once, and from then on every
+change to how a hit *feels* is a `.tscn` Siim edits in the inspector — no code, no recompile.
+
+```gdscript
+# anywhere in gameplay code
+Fx.spawn(&"blood_burst", global_transform)          # at a node
+Fx.spawn(&"slam_dust", Transform3D(Basis(), point)) # at a world point
+```
+
+## Siim's workflow
+
+1. **Open the gallery.** Run `fx/effect_gallery.tscn`. It loops through every effect on a sand-lit
+   stage with the effect's name on screen, so you see each one the way it reads in play.
+2. **Pick an effect** `.tscn` in `fx/` (`blood_burst`, `slam_dust`, `spark`, `hit_flash`) and open it.
+3. **Tune it in the inspector.** Everything lives on the `CPUParticles3D` node and its material:
+   - **Colour** → the node's *Color Ramp* (start colour → faded/transparent end).
+   - **Shape of the burst over time** → *Scale Amount Curve*.
+   - **Amount / Lifetime / Explosiveness / Spread / Velocity / Gravity** → the node's emission group.
+   - The viewport previews particles live as you drag — no play needed.
+4. **Save.** That's the whole edit. Re-run the gallery to compare.
+
+New effect? Add a `fx/<name>.tscn` (root = `CPUParticles3D` with `one_shot_effect.gd` attached — copy
+an existing one), then register its name → path in `fx/fx.gd`.
+
+## House style (keep effects reading as one game)
+
+**Palette** — pull from these, don't invent per-effect colours:
+
+| Role  | Start                     | End (fades to alpha 0)      |
+|-------|---------------------------|-----------------------------|
+| Blood | `#FF0505`                 | `#400000`                   |
+| Dust  | `#B89966` (tan)           | `#8C7326`                   |
+| Ember | `#FFF299` → `#FF8C1A`     | transparent                 |
+| Flash | `#FFFFFF`                 | transparent                 |
+
+**Timing budgets** — hits read best short: strike/flash effects ≤ **0.4 s**, dust/settle ≤ **1.1 s**.
+A one-shot effect frees itself `lifetime + 0.5 s` after spawn (`one_shot_effect.gd`), so keep
+`lifetime` honest.
+
+**PS1 look** — materials are **unshaded**, **alpha-blended**, **vertex-colour** (colour comes from the
+ramp), cull disabled. Prefer few, chunky, low-poly billboarded quads over dense fine particles; the
+game runs the PS1 filter over everything, so subtlety is lost anyway.
+
+**Mobile** — effects are **`CPUParticles3D`, never `GPUParticles3D`**. The web/GLES3/ANGLE target
+compiles particle shaders badly (see the notes in `player.gd`); CPU particles are just as tunable in
+the inspector and safe on-device. Keep this constraint when adding effects.
+
+## Files
+
+| File | What it is |
+|---|---|
+| `fx.gd` | The `Fx` autoload — `spawn(name, xform, parent=null)`; name→path registry |
+| `one_shot_effect.gd` | Root helper: fires the emitter(s) once, frees the scene when done |
+| `blood_burst.tscn` etc. | The effects themselves — **edit these** |
+| `effect_gallery.tscn` | Preview stage (run it to review) |
+| `_seed_placeholders.gd` | One-off generator that wrote the first-draft defaults; not used at runtime |