fix(interpreter): support exec {var}>&- fd-variable redirect syntax

[chaliy]

Summary

• Add parser support for {varname} fd-variable redirect syntax (e.g., exec {myfd}>&-)
• Added fd_var: Option<String> to Redirect AST node
• Parser detects {identifier} pattern before redirect operators via pop_fd_var()
• Interpreter resolves fd variable from shell variables in execute_exec_builtin

Test plan

• exec_fd_variable_close — close fd via variable reference
• exec_fd_variable_open — open fd via variable reference
• All 1925 bash spec tests pass
• Full test suite passes (2226+ tests)

Closes #964