Fix empty-buffer crash on '#'; CI now verifies all three PART_SELECT configs

- DisplayMessage: guard with tst r21 / breq Checkpress3 so pressing '#'
  with no characters buffered does not read uninitialized SRAM (r21=0
  wraps to 255 and displays 256 garbage entries)
- CI: assemble Lab 7 three times via sed substitution of PART_SELECT
  (1=timer LED roll, 2=keypad ASCII lookup, 3=SRAM message buffer)

Author: PreLabHomework

.github/workflows/avr-build.yml

@@ -26,7 +26,19 @@ jobs:
       - name: Assemble Lab 6 (external interrupts)
         run: avra -I "$AVRA_INC" Lab6_Interrupts/lab6_interrupts.asm
 
-      - name: Assemble Lab 7 (timers / keypad, Part 3)
+      - name: Assemble Lab 7 Part 1 (timer LED roll)
+        run: |
+          sed 's/^\.equ PART_SELECT = ./.equ PART_SELECT = 1/' \
+            Lab7_Timers_Keypad/lab7_timers_keypad.asm > /tmp/lab7_p1.asm
+          avra -I "$AVRA_INC" /tmp/lab7_p1.asm
+
+      - name: Assemble Lab 7 Part 2 (keypad ASCII lookup)
+        run: |
+          sed 's/^\.equ PART_SELECT = ./.equ PART_SELECT = 2/' \
+            Lab7_Timers_Keypad/lab7_timers_keypad.asm > /tmp/lab7_p2.asm
+          avra -I "$AVRA_INC" /tmp/lab7_p2.asm
+
+      - name: Assemble Lab 7 Part 3 (SRAM message buffer)
         run: avra -I "$AVRA_INC" Lab7_Timers_Keypad/lab7_timers_keypad.asm
 
       - name: Assemble Lab 8 Part 2 (PWM servo)

Lab7_Timers_Keypad/lab7_timers_keypad.asm

@@ -196,6 +196,8 @@ ReleaseSecond3:
     rjmp  Checkpress3
 
 DisplayMessage:
+    tst   r21                  ; any characters buffered?
+    breq  Checkpress3          ; no -- ignore '#' and keep waiting
     ldi   r16, 12
     out   TCNT0, r16
     ldi   r16, 0x05          ; start timer, prescaler 1024