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/macro/macro_box_ny.rs

@@ -459,8 +459,8 @@ struct NyChildMacroBox {
 }
 
 fn caps_allow_macro_source(ast: &ASTNode) -> Result<(), String> {
-    let allow_io = crate::config::env::env_flag("NYASH_MACRO_CAP_IO").unwrap_or(false);
-    let allow_net = crate::config::env::env_flag("NYASH_MACRO_CAP_NET").unwrap_or(false);
+    let allow_io = crate::config::env::macro_cap_io().unwrap_or(false);
+    let allow_net = crate::config::env::macro_cap_net().unwrap_or(false);
     use nyash_rust::ast::ASTNode as A;
     fn scan(n: &A, seen: &mut Vec<String>) {
         match n {