fix(hw): select AD9081 122.88 MHz VCXO DT

[tfcollins]

Summary

• pair the ZCU102 AD9081 m8_l4 BOOT.BIN with Kuiper's dedicated m8_l4_vcxo122p88 DT override
• document that the ordinary m8_l4 and m4_l8 DTBs configure the wrong 100 MHz VCXO
• make exporter profile regression tests require the exact 122.88 MHz path

Artifact verification

Decoded from the deployed Kuiper image:

adi,vcxo-frequency = 122880000
adi,pll1-clkin-frequencies = 122880000 30720000 0 0
adi,pll2-output-frequency = 3000000000
DTB sha256 = 079b28177388a579c7f273c9eb8dc015b145f6e7c90af6e587d3e0f0187a0fe4

Verification

• nox -s lint
• focused exporter tests: 2 passed
• live mini2 exporter updated and coordinator reflects exact BOOT/DT paths
• physical cold boot asserted live DT property and enumerated both axi-ad9081-rx-hpc and axi-ad9081-tx-hpc
• full VRT49 hosted hardware run in progress: https://github.com/tfcollins/vrt49/actions/runs/31312932085