--tb-debug CLI flag

Adds a hidden CLI flag, whatcable --tb-debug, that dumps the full IOThunderboltSwitch tree as plain text. Read-only, no rendering or interpretation, just a paste-friendly snapshot of every switch and port property.

This is groundwork for an upcoming Thunderbolt fabric feature that will surface what's actually negotiated over a Thunderbolt cable: link generation (TB3/4/5), lane width, hop count, and the connected device's identity. Designing the rendering layer needs samples from real Thunderbolt hardware, so a follow-up issue will ask for testers with TB docks, displays, SSDs, eGPUs, and daisy-chains to run the probe and paste the output.

If you don't have any Thunderbolt hardware, this release changes nothing visible for you. The menu bar app, regular CLI output, and existing flags are unchanged.