feat: Phase 10_b JIT implementation progress + academic paper ideas

Phase 10_b JIT Lower implementation:
- IRBuilder abstraction with NoopBuilder (emit counting) ✅
- CraneliftBuilder skeleton (feature `cranelift-jit`) ✅
- LowerCore implementation (Const/Copy/BinOp/Cmp/Branch/Ret) ✅
- Engine.compile with builder selection and JIT handle generation ✅
- JIT function table with stub functions ✅
- Basic i64 const/binop/ret emission for Cranelift
- VM execution path with NYASH_JIT_EXEC=1 support

Academic ideas and analysis:
- "Everything is Thread-Safe Box" concept
- "GC as debug tool" paradigm analysis
- GC switchable semantic equivalence documentation
- Gemini & Codex evaluation on academic paper potential
- Nyash academic papers potential themes

Current limitations:
- Return values limited to i64 (VMValue::Integer)
- Arguments not yet supported
- Compare/Branch emit not implemented
- Trap→VM fallback not implemented

🤖 Generated with Claude Code

Co-Authored-By: Claude <noreply@anthropic.com>

Cargo.toml

@@ -27,6 +27,12 @@ mir_refbarrier_unify_poc = []
 # llvm = ["dep:inkwell"]
 # Optional: modular MIR builder (off by default)
 mir_modular_builder = []
+cranelift-jit = [
+  "dep:cranelift-codegen",
+  "dep:cranelift-frontend",
+  "dep:cranelift-module",
+  "dep:cranelift-jit"
+]
 
 [lib]
 name = "nyash_rust"
@@ -128,6 +134,12 @@ wasm-bindgen = "0.2"
 console_error_panic_hook = "0.1"
 js-sys = "0.3"
 
+# Cranelift JIT (optional; enabled via feature "cranelift-jit")
+cranelift-codegen = { version = "0.103", optional = true }
+cranelift-frontend = { version = "0.103", optional = true }
+cranelift-module = { version = "0.103", optional = true }
+cranelift-jit = { version = "0.103", optional = true }
+
 # WASM backend dependencies (Phase 8) - optional for faster builds
 wabt = { version = "0.10", optional = true }
 wasmtime = { version = "35.0.0", optional = true }