Debug bookmark: hub enumeration fixed (NOT a release, carries the kbd_diag trace)

Marks the commit where every hub-enumeration issue found via the trace instrument
is resolved and confirmed on hardware (RPi compound hub, Terminus 1a40:0101 hub
with HyperX / Genius keyboards):

  - retry a silent enumeration control transfer instead of tearing down (eb1eee4)
  - do not suspend the hub between reset and enable (786bd53)
  - poll all hub ports in parallel so a later port is not starved (8ba99a1)
  - ignore phantom connection-changes on a cheap hub's empty ports (6fa7b19)
  - keep an empty hub instead of re-enumerating it forever (34f9f1f)

This build STILL contains the kbd_diag trace instrument (WH_CMD_TRACE, the
readout "Enum trace" panel) and reports firmware 2.3. It is a debug bookmark only.
Next: strip kbd_diag, rebuild, run TESTING.md, then stamp 2.4 and tag logger-v2.4.