ePMP devices were not reporting firmware_version because the YAML's
cambiumCurrentuImageVersion.0 OID returns empty on ePMP3000 firmware.
Add detect_version/1 to the Epmp vendor module with two-OID fallback:
cambiumCurrentSWInfo.0 (verbose, ePMP3000-friendly) → cambiumCurrentuImageVersion.0
(short, ePMP1000/2000), with a regex to extract the version triplet
from whatever the device returns.
ePMP3000 also has a sysObjectID quirk where some firmware reports the
generic NetSNMP-on-Linux ID (.1.3.6.1.4.1.8072.3.2.10) instead of the
Cambium enterprise OID, causing devices to misclassify as the linux
profile. Add a second discovery block to the epmp profile that matches
NetSNMP-OID + PREEMPT_RT armv7l kernel signature + a confirming snmpget
against the Cambium ePMP MIB tree, so detection runs in pass-2
(conditional non-generic) before linux falls through in pass-3.