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/runner/modes/llvm/fallback_executor.rs

@@ -27,7 +27,10 @@ impl FallbackExecutorBox {
         // do not silently fall back to mock.
         if crate::config::env::env_bool("NYASH_LLVM_USE_HARNESS") {
             return Err(LlvmRunError::fatal(
-                "LLVM harness requested (NYASH_LLVM_USE_HARNESS=1), but this binary was built without `--features llvm` (llvm-harness). Fix: cargo build --release --features llvm"
+                "LLVM harness requested (NYASH_LLVM_USE_HARNESS=1), but this binary was built without `--features llvm` (llvm-harness).\n\
+Fix:\n  cargo build --release -p nyash-rust --features llvm --bin hakorune\n\
+Then ensure prerequisites:\n  cargo build --release -p nyash-llvm-compiler\n  cargo build --release -p nyash_kernel\n\
+Tip: tools/run_llvm_harness.sh <program.hako>"
             ));
         }