Each platform now gets its own runner with QEMU, running concurrently.
The single-job QEMU approach built architectures sequentially.
Restores the digest+merge pattern to combine parallel builds into a
single multi-arch manifest.
Replace native arm64 runners (unavailable on free tier) with QEMU
emulation. All three image builds now trigger automatically on
`release: published`, deriving version and :latest flag from the
GitHub release event. workflow_dispatch retained for manual rebuilds.
