applyPaneExits() only applied the manager's directory when it returned a
value, so after a successful read a session that was split into several
panes, or was no longer on the socket, kept its stale cwd. The manager
already keeps its map across a failed or empty read, so the server now
applies its answer as given, undefined included, and setCwd() broadcasts
the clear once.
A blank #{pane_current_path} on a single-pane session (tmux could not read
the process cwd that tick) now carries the previous entry forward in
derivePaneCurrentPaths(), so only a split or unlisted session clears.
The server tests now drive the real TmuxManager with only the tmux read
faked. docs/api-reference.md documents SessionState.cwd.
Aamer Akhter
54c1692350