With on_load now running on every (re)load, on_ready was ambiguous. on_attach
fires ONCE when the node + script first attach to the tree (never on reload);
on_load fires after it AND on every hot reload. Migration: rename on_ready ->
on_attach in register blocks + function names.

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