docs: pin OEM kernel to 6.14 stream

[MioYuuIH]

Summary

• replace the unavailable exact OEM kernel ABI package in the README with the 6.14 OEM stream meta package
• update PXE rootfs package lists to use the 6.14 OEM image/header stream meta packages
• keep installation guidance on the 6.14 kernel stream while avoiding repeated updates for 6.14.0-10xx ABI package rotations

Verification

• rg "6.14.0-1018-oem|6.14.0-1020-oem|linux-oem-24.04|linux-headers-oem-24.04|linux-oem-6.14|linux-image-oem-6.14|linux-headers-oem-6.14" README.md deploy/ansible
• GIT_MASTER=1 git diff --check
• python3 -c 'import yaml, sys; [yaml.safe_load(open(path, encoding="utf-8")) for path in sys.argv[1:]]; print("yaml ok")' deploy/ansible/playbooks/pb-pxe-controller.yml deploy/ansible/roles/pxe_controller/defaults/main.yml
• apt-cache policy linux-oem-6.14 linux-image-oem-6.14 linux-headers-oem-6.14

Closes #167