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>

src/jit/mod.rs

@@ -0,0 +1,6 @@
+//! JIT subsystem: Cranelift-based JIT manager and lowering stubs
+
+pub mod manager;
+pub mod engine;
+pub mod lower;
+pub mod r#extern;