This refactors JIT code generation by:

1. Simplifying RVOP macro from 3 to 2 parameters, removing now-unused DSL-based GEN blocks from rv32_template.c. This eliminates ~1300 lines of dead code that was obsoleted when rv32_jit.c was added (commit 9cd48ea).
2. Introducing JIT helper macros in rv32_jit.c to reduce duplication:
   • GEN_BRANCH/GEN_CBRANCH for conditional branches
   • GEN_ALU_IMM/GEN_ALU_REG for arithmetic operations
   • GEN_SHIFT_IMM/GEN_SHIFT_REG for shift operations
   • GEN_SLT_IMM/GEN_SLT_REG for set-less-than operations
   • GEN_LOAD/GEN_STORE for memory operations with MMIO support
3. Adding symbolic constants (JCC_*) in jit.h for jump condition codes, improving cross-architecture clarity.
4. Removing the deprecated gen-jit-template.py that was no longer used after the manual JIT handler approach was adopted.

The interpreter and JIT now have clear separation of concerns:

• rv32_template.c: Pure interpreter semantics
• rv32_jit.c: Native code generation with helper macros

Close #511

Summary by cubic

Refactors Tier-1 JIT codegen to use shared helper macros and symbolic jump codes, simplifying the interpreter API and clearly separating interpreter semantics from native code generation. This reduces duplicated code and improves cross-architecture clarity (x86-64 and Arm64).

• Refactors
  • Simplified RVOP from 3 to 2 parameters; removed obsolete DSL GEN blocks in rv32_template.c.
  • Added JIT helper macros in rv32_jit.c (branches, ALU imm/reg, shifts imm/reg, slt imm/reg, load/store with MMIO).
  • Introduced JCC_* constants in jit.h for condition codes used by both architectures.
  • Removed deprecated tools/gen-jit-template.py.
  • Added docs/codegen.md covering tiers, emit API, register allocation, and MMIO handling.
  • Established clear file roles: rv32_template.c for interpreter; rv32_jit.c for native codegen.

^{Written for commit d8129e5. Summary will update on new commits.}