feat: try-catch-finally 异常处理

语法: try { ... } catch (e) { ... } finally { ... }
catch 和 finally 都是可选的，但至少需要一个。throw expr 抛出任意值。

- Lexer: 新增 try, catch, finally, throw 关键字
- AST: Stmt::Try { body, catch_var, catch_body, finally_body } + Stmt::Throw(Expr)
- Parser: try/catch/finally/throw 语句解析，更新 synchronize()
- Opcode: Throw = 46，FunctionProto 新增 exception_handlers 表
- ExceptionHandler: { try_start, try_end, catch_ip, catch_slot, finally_ip }
- Compiler: compile_try 将 finally 在成功/异常两条路径各内联一次
- VM: unwind() 栈展开 + find_handler() 处理器搜索
- RuntimeError（除零、未定义变量等）在 try 块内自动转为可捕获异常
- LSP: 补全关键字列表追加

已知限制: return/break/continue 在 try-finally 内部不会先执行 finally

16 个新测试，全部 343 个测试通过。

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

aster-core/src/lexer/lexer.rs

@@ -360,6 +360,10 @@ impl Lexer {
             "true" => TokenKind::True,
             "false" => TokenKind::False,
             "nil" => TokenKind::Nil,
+            "try" => TokenKind::Try,
+            "catch" => TokenKind::Catch,
+            "finally" => TokenKind::Finally,
+            "throw" => TokenKind::Throw,
             _ => TokenKind::Identifier(s),
         };