A loaded world starts around the player first, the rest a few areas a tick (t21)

- Areas within 150 grains of the bot start at once; the others 12 a tick, nearest first, and any
  the bot flies toward start straight away. A Huge world is fully started after 17 s.
- Huge world, first 600 ticks: average 15.2 -> 5.5 ms/tick, worst 79.7 -> 16.2.
- New test/t_load.js.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/DESIGN.md

@@ -358,6 +358,12 @@ above, including:
   still sleep. Through burning fuel sinking stays at the old slow pace: furnace beds are tuned to
   it, and dirt heaped on a burning pile must smother it into charcoal rather than sink through.
 
+- **A loaded world starts around the player first (t21, 2026-10-25).** Loading used to start every
+  area of the world at once, and a Huge world then took 15 ms a tick for its first 10 seconds,
+  with ticks up to 80 ms. Now the areas within 150 grains of the bot start at once, the rest 12 a
+  tick, nearest first (a Huge world is fully started after 17 s), and flying toward areas still
+  waiting starts them. Nothing is skipped: the same settling happens, spread out.
+
 ## Ideas not built yet
 
 - **Concrete**, as a stronger wall material from rubble.