GeezerVison

why: The per-window pane-construction body was inlined in
ClassicWorkspaceBuilder.build(), leaving no override point for a builder
that wants a different pane-creation strategy without duplicating the
window iteration, plugin hooks, focus handling, and build events.

what:
- Move the per-window pane loop into an overridable _build_window()
  that returns the window's focus pane
- Have build() call _build_window() and keep on_window_create,
  config_after_window, after_window_finished, focus, and window_done
  unchanged around it
- Add a doctest for the new seam

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

why: The new opt-in builder needs user-facing guidance and an API
reference page, and the change should be discoverable in the changelog.

what:
- Add a Concurrent builder section to the custom-workspace-builders
  guide and list it under Choosing an approach and Reference
- Add the concurrent autodoc page and link it from the builder index
  grid and toctree
- Add a What's new CHANGES entry under 1.74.0