feat(load): phase-1 preload gate + eligible-pick pool growth

Gate the universe on one clip per altitude + connecting morphs (~126 MB, ~10-20s),
then grow the pool in the background. The random pick (pickPoolClip/pickRandomMember)
is restricted to fully-loaded clips — base + connecting morphs both ways cached
(HEFPreload.eligibleDestinations) — so a transition never stalls on un-downloaded
media and new clips become reachable only once fully loaded. Pure planner in
preload.js (+ node tests).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tools/build_static.py

@@ -36,8 +36,8 @@ from simulator.app import MEDIA_DIR, create_app
 
 STATIC = Path(__file__).resolve().parent.parent / "simulator" / "static"
 # Frontend files that ship; everything else in static/ (author*, review*) is dev-only.
-PUBLIC_ASSETS = ["index.html", "app.js", "scrub.js", "i18n.js", "alteration.js", "style.css",
-                 "credits.html", "credits.js", "about.html", "flash.js"]
+PUBLIC_ASSETS = ["index.html", "app.js", "scrub.js", "i18n.js", "alteration.js", "preload.js",
+                 "style.css", "credits.html", "credits.js", "about.html", "flash.js"]
 
 
 def _bake_api(app_dir: Path) -> dict:
@@ -84,7 +84,8 @@ def _version_assets(app_dir: Path) -> None:
     # = guaranteed-fresh fetch on a normal reload. The HTML itself is served no-cache
     # (_headers), but Cloudflare Pages caches .js/.css by type (max-age=14400) and
     # ignores _headers for them — so without this, returning visitors run stale JS.
-    assets = ["app.js", "scrub.js", "i18n.js", "alteration.js", "style.css", "credits.js", "config.js"]
+    assets = ["app.js", "scrub.js", "i18n.js", "alteration.js", "preload.js", "flash.js",
+              "style.css", "credits.js", "config.js"]
     tok = {}
     for a in assets:
         p = app_dir / a