refactor: separate core and optimizer images

[frahlg]

Summary

• remove Python/CVXPY and optimizer source from the official core image
• run the static core binary on Alpine with only CA certificates, timezone data, Lua drivers and web assets
• keep Python/CVXPY exclusively in Dockerfile.optimizer, reached through the existing Unix-socket contract with the Go fallback retained
• build Go on BUILDPLATFORM and cross-compile for TARGETARCH instead of running the Go toolchain under QEMU
• add a container-boundary regression test to make verify and the module-boundary CI job

Beta finding

The v1.2.0 beta correctly exposed a remaining architecture/performance bug: its optimizer image built in 1m43s, but the core image also rebuilt and embedded the optimizer and took 14m05s (13m14s in build/push alone). That candidate will not be promoted to stable.

Verification

• make verify
• make verify-all
• cold local multiarch build (linux/amd64,linux/arm64) — ~33s
• runtime container starts and passes the exact BusyBox-wget /api/health check
• core image contains no Python or /opt/venv
• ARM64 core image: 24.8 MB locally; old v1.2.0-beta.1 ARM64 manifest: ~257 MB compressed layers
• Changesets proposes the next minor release