Move the FX placeholder seeder into tools/

_seed_placeholders.gd is a one-off generator, not a shipped effect — it belongs
with the other headless tooling (bake_crowd_sheet, preview_*), not in the fx/
runtime dir Siim edits. Rename it to tools/seed_fx_placeholders.gd (matching the
dir's verb_noun convention) and repoint the two doc-comment references.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Pq1VX2o1ZfiNRdCjD294wf

fx/fx.gd

@@ -4,7 +4,7 @@ extends Node
 ## Autoload singleton. Usage:  Fx.spawn(&"blood_burst", hit_transform)
 ##
 ## Scenes are loaded lazily (not preloaded) so this autoload compiles and boots even before the
-## placeholder .tscn are seeded — see fx/_seed_placeholders.gd.
+## placeholder .tscn are seeded — see tools/seed_fx_placeholders.gd.
 
 const _PATHS: Dictionary = {
 	&"blood_burst": "res://fx/blood_burst.tscn",