fix(screen): CHROUT prints PETSCII $E0-$FF as screen codes $60-$7E and pi

PETSCII $E0-$FE became the reverse screen codes $A0-$BE. A C64 prints them as
$60-$7E, as it does $A0-$BE, and $FF, pi, as $5E, as it does $DE (C64
Programmer's Reference Guide, appendices B and C).

src/kernal/screen/chrout_to_screen_code.s

@@ -9,14 +9,18 @@
 
 chrout_to_screen_code:
 
-	; Codes $C0-$DF become $40-$5F
-	cmp #$E0
-	bcs @1
+	; Codes $C0-$FE become $40-$7E, and $FF (pi) becomes $5E, as $DE does
+	; (C64 PRG p380-381)
+	cmp #$FF
+	beq @pi
 	cmp #$C0
 	bcc @1
 	
 	and #$7F
 	rts                                ; not high char
+@pi:
+	lda #$5E
+	rts
 @1:
 	; Range $20-$3F is unchanged
 	cmp #$40