re: withdraw "the speed cannot be used", and close the world-unit item

Last iteration I ruled the speed route out because the emulator is not real-time
so the wall-clock denominator is unknown.  Wrong: the game prints its own clock,
and flight-speed-law.md had already used it -- mission TIME across a wall interval
gives 1.26x, turning 443.6 units/wall-second into 352 per game-second against a
HUD 350.  Withdrawn in place, with the reasoning, rather than deleted.

Closes the backlog item: one world unit is one metre, by two independent routes.

docs/re/BACKLOG.md

@@ -86,6 +86,15 @@ See [`mission-freeze-heap-exhaustion.md`](mission-freeze-heap-exhaustion.md).
   between tool calls.** Sequential tool calls *within* one turn are fine; it is
   ending the turn that kills it.
 
+✅ **The world-unit measurement is DONE — one world unit is one metre.** Two
+independent routes: the `mapmesh_box_500km` collision hull is a cube of exactly
+500 000.0 units per axis (and the other box mesh exactly 100 000.0), and
+[`flight-speed-law.md`](flight-speed-law.md) gives 352 units per game-second
+against a HUD reading of 350. The `CollisionSet` objection that forced the 🟡 is
+dismantled in [`structures/mcol-collision.md`](structures/mcol-collision.md): it
+read the four-member `rob_` family (which contains a 50 km "player fighter") as
+if it were hull sizes, and 133 m is the 7th smallest of 78 `rou_` meshes.
+
 ❔ **The world-unit measurement is still not made** — but the blocker under it is
 now GONE. The entity scan works: see
 [`entities-live-roster.md`](entities-live-roster.md), **13 unit definitions and 42