Separate fast release builds from slow max-perf builds to optimise runner
utilisation and provide quicker feedback. Release builds complete first with
standard optimisations, followed by Haswell-optimised dragrace builds once
the safe builds pass successfully.
Extract build logic into focused composite actions for better log visibility
in Forgejo UI. Split monolithic build action into prepare-docker-build,
inline docker build step, and upload-docker-artifacts to ensure each phase
completes independently and shows logs immediately.
Creates separate manifests at each stage to avoid waiting for all builds
before publishing.
