feat(debos): install Radxa vendor kernel for RK3588S2 #82

Problem

The current radxa-rock5c.yaml recipe installs linux-image-arm64 (generic Debian ARM64 kernel). This boots on QEMU and gives a working baseline image, but does not provide full hardware support for the RK3588S2 SoC:

• NPU (/dev/rknpu) not available
• RGA 2D accelerator not available
• MIPI-CSI camera not available
• Hardware video encode/decode not available
• Other Rockchip-specific peripherals missing

Goal

Replace linux-image-arm64 with the Radxa vendor kernel for Rock 5C, so the OS image can drive all hardware features needed by pai-engine.

Acceptance Criteria

• Identify the correct vendor kernel package (Radxa APT repo or prebuilt .deb)
• Add the Radxa APT source to base-arm64.yaml or radxa-rock5c.yaml
• Replace linux-image-arm64 with the vendor kernel package in radxa-rock5c.yaml
• Verify /dev/rknpu is present after boot on Radxa Rock 5C
• Update operating-system.mdx to reflect the vendor kernel once shipped

References

• os/recipes/machines/radxa-rock5c.yaml — current kernel install
• docs/src/content/docs/architecture/operating-system.mdx — documents the kernel layer
• Radxa package repo: https://github.com/radxa-pkg