arm64 / Raspberry-Pi thin appliance (ADR-103 Stage 2 cont.)

[aaronsb]

Continue ADR-103 Stage 2 to an arm64 / Raspberry-Pi thin appliance. The x86 qcow2/OVA path is done (#510); arm64 is gated on:

• AGE arm64: Postgres+AGE image is pinned to an amd64 SHA digest — confirm/repin an arm64 variant.
• Garage arm64: verify dxflrs/garage publishes arm64.
• QEMU bake under emulation: confirm bake_embedding_models.py (loads nomic text+vision) completes building the arm64 image under emulation.
• Pi RAM budget: validate Postgres+AGE + Garage + API + nomic (~400MB loaded) fit in 4 GiB under real ingestion.
• build-appliance.sh: arm64 target + flashable .img output.

Refs ADR-103, #510.