feat(load[spinner]): wire progress spinner into tmuxp load command

why: Users need visual feedback during workspace builds, especially for
sessions with many windows or long before_script executions.
what:
- Add _silence_stream_handlers() to suppress StreamHandler during spinner
- Add _dispatch_build() extracting shared build/attach/error logic
- Wire Spinner.on_build_event and add_output_line to builder callbacks
- Add --progress-format / TMUXP_PROGRESS_FORMAT for preset or custom format
- Add --progress-lines / TMUXP_PROGRESS_LINES for panel height control
- Add --no-progress / TMUXP_PROGRESS=0 to disable spinner entirely
- Emit persistent success line with checkmark after successful build
- Stop spinner before interactive prompts (TMUX switch, error recovery)

Author: tony