WorkspaceBuilder: add an opt-in parallel builder

[tony]

Summary

Add a separate opt-in builder that prioritizes faster workspace construction while keeping the classic builder as the compatibility default.

Related: #1053, #1054, #1056, #1058.

Proposed behavior

• Implement a parallel builder selected explicitly through the builder mechanism introduced in the base work.
• Keep the classic builder as the default for existing workspaces and dependency-sensitive configurations.
• Allow the parallel builder to create pane/window structure and observe readiness with less serialized waiting where the builder contract allows it.
• Document that pane commands are not guaranteed to prepare later pane split-time inputs such as start_directory, shell, window_shell, pane environment, or other split-time state.
• Recommend classic builder behavior or earlier setup phases such as before_script / shell_command_before for configs that rely on strict side effects.
• Keep this separate from the experimental chain-builder work while allowing the protocol to learn from Experimental: pure-chain ChainWorkspaceBuilder + --builder=chain #1054.

Acceptance criteria

• The parallel builder is opt-in and not selected by existing workspace files.
• Tests cover faster grouped creation behavior and the documented relaxed dependency guarantees.
• Compatibility-sensitive ordering remains covered by classic-builder tests.
• Documentation explains when to choose classic versus parallel behavior.