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/mir/control_tree/normalized_shadow/common/expr_lowerer_box.rs

@@ -159,6 +159,7 @@ impl NormalizedExprLowererBox {
                     }
                 }
                 UnaryOperator::BitNot => Some(OutOfScopeReason::UnsupportedOperator),
+                UnaryOperator::Weak => Some(OutOfScopeReason::UnsupportedOperator), // Phase 285W-Syntax-0
             },
 
             ASTNode::BinaryOp { operator, left, right, .. } => {
@@ -232,6 +233,7 @@ impl NormalizedExprLowererBox {
                     }
                 }
                 UnaryOperator::BitNot => Some(OutOfScopeReason::UnsupportedOperator),
+                UnaryOperator::Weak => Some(OutOfScopeReason::UnsupportedOperator), // Phase 285W-Syntax-0
             },
 
             ASTNode::BinaryOp { operator, left, right, .. } => {
@@ -387,6 +389,7 @@ impl NormalizedExprLowererBox {
                 Ok(Some(dst))
             }
             UnaryOperator::BitNot => Ok(None),
+            UnaryOperator::Weak => Ok(None), // Phase 285W-Syntax-0: Not supported in normalized lowering
         }
     }