[Maintenance] Use *_CELS to denote the number of distinct animation sprites

Which are typically lower than the amount of *_FRAMES they're shown in.

Part of P0074, funded by Myles.

CONTRIBUTING.md

@@ -196,6 +196,9 @@ binary, as compared using [mzdiff].** The only allowed exceptions are:
 
 * ASM file extensions: `.asm` if they emit code, `.inc` if they don't
 * Macros defining the number of instances of an entity: `<ENTITY>_COUNT`
+* Macros defining the number of distinct sprites in an animation: `*_CELS`
+* Frame variables counting from a frame count to 0: `*_time`
+* Frame variables and other counters starting from 0: `*_frames`
 * Functionally identical reimplementations or micro-optimizations of
   master.lib functions: `z_<master.lib function name>`
 

th05/gaiji/gaiji.inc

@@ -11,5 +11,5 @@ ga_RETURN_KEY_0 = ga_RETURN_KEY + 0
 ga_RETURN_KEY_1 = ga_RETURN_KEY + 1
 ga_RETURN_KEY_2 = ga_RETURN_KEY + 2
 ga_RETURN_KEY_3 = ga_RETURN_KEY + 3
-ga_RETURN_KEY_FRAMES = 4
+ga_RETURN_KEY_CELS = 4
 ; ----------

th05_maine.asm

@@ -776,7 +776,7 @@ loc_A962:
 		push	point_15004.y
 		mov	ax, di
 		shr	ax, 3
-		and	ax, ga_RETURN_KEY_FRAMES - 1
+		and	ax, ga_RETURN_KEY_CELS - 1
 		add	ax, ga_RETURN_KEY
 		push	ax
 		push	15