Add COS

src/basic/math/e264.cos_FAC1.s

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+;; #LAYOUT# STD *       #TAKE
+;; #LAYOUT# *   BASIC_0 #TAKE
+;; #LAYOUT# *   *       #IGNORE
+
+;
+; Math package - cosine of FAC1 in radians
+;
+; Adds pi/2 to FAC1 and falls through to the sine function
+;
+; See also:
+; - [CM64] Computes Mapping the Commodore 64 - page 210
+; - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
+; - https://codebase64.org/doku.php?id=base:kernal_floating_point_mathematics
+;
+
+COS:
+    ldy #>const_HALF_PI
+    lda #<const_HALF_PI
+    jsr add_MEM_FAC1
+    
+    ; FALLTHROUGH to SIN

src/basic/math/e264.interop

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+This is an important floating point math routine, documented in many places.
+
+Usage example:
+- https://www.lemon64.com/forum/viewtopic.php?t=10492