USB report: ~3 s per power change; flapping-charger edge case

Summarises the measured cost as roughly 3 s blank per PD state change,
with keep-awake in the app handling sleep, and notes that a marginal
charger renegotiating PD repeatedly would cut the link each time.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/impl/reports/USB-TABLET-LINK.md

@@ -126,6 +126,9 @@ Three PD unplug/replug cycles while pinging the tablet every 0.5 s:
 - So with the tablet awake, the hub's PD changes cost **~3.5 s on unplug** (full detach) and **~2.2 s on plug-in** (gadget reconnect only).
   Everything beyond that came from tablet sleep.
 - To the Orin, a sleeping tablet is indistinguishable from a dead link, except by heartbeat.
+- Rule of thumb: **about 3 s blank per power-state change**. The app keeping the tablet awake (full UI or an overlay widget) takes sleep off the table.
+- Known edge case, not a blocker: an underpowered or marginal charger that keeps falling back between PD and 5 V/1 A would cut the link on
+  every renegotiation, possibly every few seconds. Use a solid PD charger. If the link ever flaps periodically, suspect the charger first.
 
 - At ignition, accessory power drops during cranking, so a start is likely to produce both edges back to back (not tested in the car yet).