re: unit definitions are parsed at stage load, not on wave spawn

Counted the definition objects at three points in Stage 02 -- just after
take-off, ~12 min into combat, and after GAME OVER: 14 objects, same 14 IDs
each time. Capturing a stage therefore costs one load and one snapshot; the
mission does not have to be played or survived.

docs/re/structures/unit-struct-runtime.md

@@ -219,4 +219,10 @@ tutorial adds its own units; `unit_runtime.py` unions any number of snapshots
 and re-solves, and more units directly promote the 🟡 bindings to ✅ by adding
 distinct values.
 
+**A stage's whole unit set is parsed at load, not as waves spawn** — checked by
+counting the definition objects at three points in Stage 02: immediately after
+take-off, ~12 minutes in mid-combat, and after GAME OVER. 14 objects, the same
+14 IDs, every time. So capturing a stage costs one load and one snapshot; there
+is no need to play it, and no need to survive it.
+
 Stages captured so far: `Ttrl` (BASIC CONTROLS), Stage 02.