[Reverse-engineering] [th04] Discover the cause behind the Kurumi crash on Easy

Major thanks to Colin Douglas Howell, who looked into the issue and
provided instructions for reliably reproducing it:

	https://github.com/nmlgc/rec98.nmlgc.net/issues/2

Part of P0189, funded by Arandui and Lmocinemod.

th04/main/bullet/types.h

@@ -20,6 +20,9 @@ typedef enum {
 	BG_RANDOM_CONSTRAINED_ANGLE_AIMED = 0x1D,
 
 	// Ring out of [count] bullets, ignoring [delta].
+	// ZUN bug: Must be ≥1 going into the bullets_add_*() functions to prevent
+	// divisions by zero ("Divide error"). This is exactly what causes the
+	// crash during Kurumi when playing on Easy and with minimum rank.
 	BG_RING = 0x26,
 	BG_RING_AIMED = 0x2C,