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>

docs/language/assets/assets-total.md

@@ -0,0 +1,14 @@
+---
+id: assets-total
+name: Assets.total
+category: assets
+kind: namespace-method
+tokens: Assets.total
+sig: Assets.total() -> int
+tip: How many assets are enqueued.
+order: 6
+ns: Assets
+member: total
+---
+
+Returns the total number of assets enqueued with <a href="assets-enqueue"><code>Assets.enqueue</code></a> — the denominator for a progress bar.