Previously the libghostty-vt stream handler dropped .report_pwd as a
no-op, so embedders never saw shell-reported cwd changes and the
terminal's pwd field was never populated from escape sequences.
Wire the action to setPwd and expose a pwd_changed callback analogous
to title_changed via GHOSTTY_TERMINAL_OPT_PWD_CHANGED. The payload is
passed through unparsed; embedders read it with ghostty_terminal_get
and decode any URI scheme themselves.
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