Power ETH_PHY_POWER down for 200 ms and back up 200 ms before ETH.begin()
so the PHY always gets a true cold start. A warm PHY left in a stale state
(reproduced on Theengs Bridge v1.1 with PSRAM enabled after an NVS erase)
can flap the link until a manual power cycle: it reports up for one poll,
then down for several seconds, forever, so DHCP never completes. The short
reset the Ethernet driver performs on its own does not clear that state; a
power cycle with a settle window before begin() does. On a hardware-in-the-
loop bench the same provocation went from flapping every trial to flapping
in none of them (4/4).
Enabled via -DETH_PHY_COLD_CYCLE, only active when ETH_PHY_POWER is a valid
pin, at the cost of ~400 ms added boot time. Set by default on both Theengs
Bridge environments.
Co-authored-by: Florian <1technophile@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
