start.sh builds via 'make build' (platform auto-detected) then starts
services detached with 'docker compose up -d'.
Makefile gains build/build-<platform> targets that build images without
starting, mirroring the existing up/<platform> targets.
Remove hardcoded platform from compose file so plain 'make up' (or
'docker compose up --build') always builds natively for the host.
Explicit targets (up-arm64, up-armv7, etc.) set DOCKER_DEFAULT_PLATFORM.
Dockerfile now uses BuildKit TARGETARCH/TARGETVARIANT to pick the Rust
cross-compilation target automatically. The build stage always runs on
the host platform for speed.
Makefile provides named targets:
make up-amd64 # Mac Intel / Linux desktop
make up-arm64 # Mac M1/M2/M3, Pi 4/5 (64-bit OS)
make up-armv7 # Pi 2/3/4 (32-bit OS)
make up-armv6 # Pi Zero / Pi 1