Implement BRGE and BRLT signed branch instructions #10

Currently, our Tiny8 CPU simulator supports unsigned branch instructions such as BRCS and BRCC.
To fully support signed comparisons, we need to implement the BRGE (Branch if Greater or Equal,
Signed) and BRLT (Branch if Less Than, Signed) instructions.

Tasks:

• Implement BRGE to branch when Rd >= Rr for signed integers.
• Implement BRLT to branch when Rd < Rr for signed integers.
• Ensure the instructions correctly use the S flag (S = N xor V) from SREG.
• Add unit tests demonstrating signed comparisons.

References:

• [1] AVR® Instruction Set Manual