luna-jit-helpers vs luna-core vs luna-jit-llvm

Shared `luna_jit_*` extern-C runtime helpers + per-thread JIT_VM/JIT_CL TLS slots + `enter_jit` RAII rebind for the goliajp/luna Lua runtime. Used by both `luna-jit` (Cranelift backend) and `luna-jit-llvm` (LLVM alt backend) so the helper symbol table and TLS discipline stay single-source-of-truth across JIT backends. Internal infrastructure crate — embedders should depend on `luna-jit`, not this crate directly.

Pure-Rust Lua runtime (interpreter only, zero third-party dependencies). The JIT-equipped variant lives in the `luna-jit` crate.

LLVM 18 + inkwell 0.9 alternative JIT backend for the goliajp/luna Lua runtime. Lives alongside `luna-jit` (Cranelift); backend selection at runtime via the `LUNA_JIT_BACKEND=llvm` env var. Internal alt-backend crate — embedders should depend on `luna-jit` and enable the `llvm-jit` feature, not depend on this crate directly. Requires `LLVM_SYS_181_PREFIX` pointing at an LLVM 18.1.x install at build time.