feat(phase285): Complete weak reference implementation (VM + LLVM harness)

Phase 285LLVM-1.1 to 1.4 + weak reference infrastructure:

**LLVM Harness** (Phase 285LLVM-1.x):
- 285LLVM-1.1: User Box registration & debug output
- 285LLVM-1.2: WeakRef basic operations (identity deferred)
- 285LLVM-1.3: InstanceBox field access (getField/setField)
- 285LLVM-1.4: print Handle resolution (type tag propagation)

**VM Runtime** (nyash_kernel):
- FFI functions: nyrt_weak_new, nyrt_weak_to_strong, nyrt_weak_drop
  (crates/nyash_kernel/src/lib.rs: +209 lines)
- WeakRef plugin invoke support
  (crates/nyash_kernel/src/plugin/invoke.rs: +250 lines)
- weak_handles.rs: WeakRef handle registry (NEW)

**LLVM Python Backend**:
- WeakRef instruction lowering (weak.py: NEW)
- Entry point integration (entry.py: +93 lines)
- Instruction lowering (instruction_lower.py: +13 lines)
- LLVM harness runner script (tools/run_llvm_harness.sh: NEW)

**MIR & Runtime**:
- WeakRef emission & validation
- MIR JSON export for weak instructions
- Environment variable support (NYASH_WEAK_*, HAKO_WEAK_*)

**Documentation**:
- CLAUDE.md: Phase 285 completion notes
- LANGUAGE_REFERENCE_2025.md: Weak reference syntax
- 10-Now.md & 30-Backlog.md: Phase 285 status updates

Total: +864 lines, 24 files changed

SSOT: docs/reference/language/lifecycle.md
Related: Phase 285W-Syntax-0, Phase 285W-Syntax-0.1

src/mir/builder/lifecycle.rs

@@ -82,13 +82,17 @@ impl super::MirBuilder {
             }
             ASTNode::BoxDeclaration {
                 name,
+                fields,          // Phase 285LLVM-1.1: Extract fields
                 methods,
                 is_static,
                 ..
             } => {
                 if !*is_static {
-                    self.comp_ctx.user_defined_boxes.insert(name.clone());
+                    // Phase 285LLVM-1.1: Register instance box with field information
+                    self.comp_ctx.register_user_box_with_fields(name.clone(), fields.clone());
                 } else {
+                    // Static box: no fields
+                    self.comp_ctx.register_user_box(name.clone());
                     for (mname, mast) in methods {
                         if let ASTNode::FunctionDeclaration { params, .. } = mast {
                             self.comp_ctx
@@ -223,7 +227,8 @@ impl super::MirBuilder {
                             }
                         } else {
                             // Instance box: register type and lower instance methods/ctors as functions
-                            self.comp_ctx.user_defined_boxes.insert(name.clone());
+                            // Phase 285LLVM-1.1: Register with field information for LLVM harness
+                            self.comp_ctx.register_user_box_with_fields(name.clone(), fields.clone());
                             self.build_box_declaration(
                                 name.clone(),
                                 methods.clone(),
@@ -534,6 +539,9 @@ impl super::MirBuilder {
         // main 関数スコープの SlotRegistry を解放するよ。
         self.comp_ctx.current_slot_registry = None;
 
+        // Phase 285LLVM-1.1: Copy user box declarations to module metadata for LLVM harness
+        module.metadata.user_box_decls = self.comp_ctx.user_defined_boxes.clone();
+
         Ok(module)
     }