Add active .xdf monitoring for near real-time timeline updates

[Copilot]

This change enables timeline updates from an actively written LabRecorder .xdf instead of relying only on new-file discovery. It reuses existing multi-XDF loading/processing paths while adding fallback metadata handling for in-progress files.

• Active recording selection + metadata fallback

  • Added discovery helpers to:
    • select the most recent .xdf when multiple candidates exist
    • derive reference datetime from file metadata when close-time header fields are missing (falling back to file mtime for open/in-progress files)

• Active .xdf polling in TimelineBuilder

  • Extended refresh config with:
    • enable_active_xdf_monitoring (default True)
    • active_xdf_poll_interval_seconds (default 10.0)
  • Added a fixed-interval QTimer poll loop.
  • Tracks active file signature (size, mtime_ns) to detect incremental writes.

• In-place datasource refresh (no duplicate tracks)

  • On active file growth/change, rebuilds datasource payloads and updates existing timeline datasources in place (intervals_df, detailed_df, raw/lab metadata), then emits datasource change signals so renderers refresh.
  • Keeps existing discovery behavior for newly created XDF/video files.
  • Uses a narrower refresh path when only one loaded XDF exists.

• Shared fallback integration

  • Wired stream processing to use the new reference-datetime fallback helper so open-file loading paths consistently handle missing close-time metadata.

builder.set_refresh_config(
    xdf_discovery_dirs=[lab_recorder_dir, log_dir],
    n_most_recent=15,
    enable_active_xdf_monitoring=True,
    active_xdf_poll_interval_seconds=10.0,
)

[CommanderPho]

It launches but when there's an actively recording .xdf file the timeline GUI seems to freeze upon launch.

• Determine the cause of this.
  @copilot