feat(assets): #82 incremental asset preloading + loading-scene pattern

Assets loaded synchronously in Boot stalled the first frame(s). Adds an
Assets.* preload queue over the #81 atlas: Assets.enqueue(name, path) queues a
named image without loading it, Assets.pump(max) loads up to max per frame
(returns how many), and Assets.total/loaded/ready/progress (0..100) drive a
progress bar. A loading scene pumps a few per frame, draws Assets.progress(),
and becomes the play scene once Assets.ready() — the deterministic, no-threads
form of async preloading (work spread across frames; same enqueue+pump order
loads identically every run). Loaded assets are reachable by name via
Assets.get / Sprite.named.

Example preload (enqueue 3, pump incrementally 0->33->66->100, ready flips, get
by name) prints 3 0 0 0 1 33 66 1 100 1. 6 docs pages. Full suite 117/0,
fixpoint holds.

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

selfhost/backend/emit_call.ludic

@@ -196,6 +196,13 @@ function emit_ns_call(ns: pointer, meth: pointer, e: Node) -> Val {
     if (meth == "image")       { bare = "atlas_image";       push(labels, "path") }
     if (meth == "load")        { bare = "atlas_image";       push(labels, "path") }   # alias
     if (meth == "get")         { bare = "atlas_named";       push(labels, "name") }    # alias of Sprite.named
+    # #82 — incremental preload queue + progress for a loading scene.
+    if (meth == "enqueue")     { bare = "assets_enqueue";    push(labels, "name"); push(labels, "path") }
+    if (meth == "pump")        { bare = "assets_pump";       push(labels, "max") }
+    if (meth == "total")       { bare = "assets_total" }
+    if (meth == "loaded")      { bare = "assets_loaded" }
+    if (meth == "ready")       { bare = "assets_ready" }
+    if (meth == "progress")    { bare = "assets_progress" }
   }
   # Camera.* — the world-space camera: a draw offset threaded through the render
   # path (runtime/native/core.ludic). set/follow move it; shake jitters it from