fix(interpreter): route exec fd redirects through VFS targets

[chaliy]

Summary

• Add persistent exec_fd_table to track fd output targets set by exec N>/path
• exec N>/dev/null now properly discards writes to fd N
• exec N>/file routes writes to the VFS file
• exec N>&M duplicates fd targets; exec N>&- closes them
• Both apply_redirections and apply_redirections_fd_table check exec_fd_table

Test plan

• Unit tests: test_exec_fd_to_dev_null, test_exec_fd_to_file
• Spec tests: exec-fd-redirect.test.sh (4 cases: dev/null, file, dup, close)
• Full test suite passes (2222 tests)

Closes #938