feat(phase285w): Implement weak x unary operator syntax

Phase 285W-Syntax-0: Migrate weak reference syntax from function call
to unary operator for consistency and clarity.

**Changes**:
- Parser: Add UnaryOperator::Weak variant and parse_unary() handling
- MIR: Lower UnaryOp::Weak to emit_weak_new() (reuses existing path)
- AST: Add Weak to UnaryOperator enum + Display/JSON support
- Tests: Migrate 8 files from `weak(x)` to `weak x` syntax
  - 7 .hako test files updated
  - 1 smoke test shell script updated
- Cleanup: Remove obsolete weak(x) parser/MIR special cases
- Docs: Update Phase 285 README

**Syntax Change**:
- Old: `local w = weak(x)`  (function call)
- New: `local w = weak x`   (unary operator)

**Validation**: All migrated phase285* smoke tests pass (4/4 relevant)

**SSOT**: docs/reference/language/lifecycle.md

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/ast.rs

@@ -261,6 +261,7 @@ pub enum UnaryOperator {
     Minus,  // -x
     Not,    // not x / !x
     BitNot, // ~x
+    Weak,   // weak x (Phase 285W-Syntax-0)
 }
 
 /// 二項演算子の種類
@@ -292,6 +293,7 @@ impl fmt::Display for UnaryOperator {
             UnaryOperator::Minus => "-",
             UnaryOperator::Not => "not",
             UnaryOperator::BitNot => "~",
+            UnaryOperator::Weak => "weak",
         };
         write!(f, "{}", symbol)
     }