Add addi-zero assembly peephole rule (Topic 13)

[zinoe-1]

Summary

Adds a new assembly-level peephole rule for Topic 13 practice: eliminate no-op addi rd, rd, 0 instructions.

Also includes a small demo fixture for hands-on experimentation.

Changes

• addi zero elimination rule in scratchv/backend/asm_peephole.py
  • Matches addi rd, rd, 0 when destination equals source
  • Removes the instruction entirely
• Unit tests in tests/test_asm_peephole.py
  • Verifies zero-immediate removal
  • Verifies non-zero addi is preserved
• Demo fixture at benchmarks/fixtures/peephole_demo.asm

Try it

python3 -m pytest tests/test_asm_peephole.py -v
python3 -m scratchv.backend.asm_peephole benchmarks/fixtures/peephole_demo.asm --report

Example optimization

Before:

li a0, 5
addi a0, a0, 3
addi a0, a0, 0
beq x0, x0, .Ldone

After:

li a0, 8  # peephole: li+addi fusion
j .Ldone  # peephole: beq zero-zero to jump

Test plan

• python3 -m pytest tests/test_asm_peephole.py (19 passed)
• Manual run on benchmarks/fixtures/peephole_demo.asm