feat(buzbee): add command hashing mode

src/buzbee.s

@@ -95,6 +95,21 @@ FULLBUF@
   jsr POUTCHAR
   bra INPLOOP
 
+; Makes a hash of four chars in BBIN starting at offset X.
+; @param X: offset in BBIN to read the four chars from
+; @return A: resulting hash
+MKCMDSUM
+  pshs b
+  ldb #4           ; Loop over four chars
+  clra             ; Initialize accumulator
+NEXTC@
+  suba BBIN_BASE,x ; Subtract current char from accumulator
+  leax 1,x         ; Next char
+  decb             ; Reduce count
+  cmpb #0          ; Are we at the end?
+  bne NEXTC@       ;  No? loop
+  rts
+
 PROMPTLINE
   fcb $0D,$0A,$25,$00 ; CR LF '%' NUL