Epic 2: Wire IR into the compilation pipeline (AST → TAC → MIPS)

Goal: IR is not a teaching demo; it actually drives codegen and optimizations.

• Define pipeline stages explicitly:
  • parse → sema → lower-to-TAC → optimize → regalloc/stack-alloc → emit MIPS
• Ensure TAC lowering covers all COOL constructs (if not already complete)
• Write a TAC-to-MIPS code generator (new backend):
  • calling convention (args, return, saved regs)
  • stack frame layout rules
  • object layout / runtime calls
  • string ops (concat, length, substr) correctness
• Keep current AST-to-MIPS backend temporarily as reference/fallback
• Add integration tests comparing TAC-path output vs AST-path output (until AST path is retired)