Cache Research image compilation before Docker packaging

[proerror77]

Change contract

When the full Prediction Markets research-image smoke is selected, compile the six release binaries in a cacheable Debian Bookworm job and make Docker package those exact binaries without recompiling Rust.

Acceptance evidence

• The binary-build job uses the repository existing Rust and sccache cache path in a Debian Bookworm environment compatible with the runtime image.
• The Docker prebuilt target contains all six expected executable entrypoints.
• Existing policy-snapshot and alpha-harness image probes pass against the prebuilt image.
• Existing image-smoke selection tests and actionlint pass.
• A warm GitHub run records the binary-build and image-package durations for comparison with the current approximately 24-minute image build.

Out of scope

ACR publish changes, cross-workflow artifact reuse, self-hosted runners, research data or database packaging, research behavior, collector deployment, and runtime cutover.

Dependency / merge order

Depends only on merged PR #217. ACR adoption, if needed, is a separate follow-up issue after this packaging target merges.

Rollout / rollback

Rollout changes only the selected CI smoke build path. The default Dockerfile target continues source-building for local compatibility. Rollback returns the CI job to the default source-building Docker target.

[proerror77]

Parent: #240. This issue owns only cacheable exact-main binary compilation and prebuilt Docker packaging; snapshot caching, mission dispatch, and ACK rollout remain separate child issues.

[proerror77]

Write ownership: this task owns branch codex/research-image-build-cache at HEAD 737e291 and the existing two-file working-tree draft. Other agents remain read-only until ownership is handed off.