why: Loading a workspace waited on each pane's shell prompt one at a
time, so a window with several panes paid the prompt-startup cost
serially even though tmux warms every pane's shell concurrently.

what:
- Add ConcurrentWorkspaceBuilder, an opt-in builder that creates a
  window's panes up front, waits for their prompts in one shared
  barrier, lays out once, then dispatches each pane's commands
- Port _pane_has_drawn_prompt and _wait_for_panes_ready (one
  shared-timeout polling loop, 10ms interval) from the source change
- Port _split_pane_reclaiming_space (reclaim layout space only on a
  no-space split failure) and the sequential fallback for panes whose
  start_directory depends on an earlier pane's command
- Respect workspace_builder_options.pane_readiness via the classic
  _pane_readiness_wait resolution; skip the barrier under never
- Register the `concurrent` entry point and export the class
- Register the module's doctests with the tmux-fixture set in conftest
- Add functional tests: multi-window/pane build, command dispatch,
  single-pass layout, readiness policy, space reclaim, sequential
  start_directory fallback