Fill release readiness from score smoke artifacts

[MaxGhenis]

Summary

• synthesize tier-level release-readiness rows from scores.json artifacts that already have runtime_smoke_loader.json
• keep full mp300k_artifact_gates.json reports authoritative when present
• treat score+smoke-only evidence as incomplete until a full gate report exists, so it can surface candidates without certifying release readiness
• derive candidate_beats_baseline from losses when older score payloads omit the boolean

Checks

• uv run --python 3.13 --extra dev ruff check src/microplex_us/pipelines/dashboard.py tests/pipelines/test_dashboard.py
• uv run --python 3.13 --extra dev pytest -q tests/pipelines/test_dashboard.py

Dashboard verification

With the shared artifact root, /Users/maxghenis/CosilicoAI/microplex-us/artifacts/microplex_dashboard_current.json reports:

• mp-120k: release_ready, loss 0.09356981757194573 vs eCPS 0.16636673113001124
• mp-152k: blocked by artifact_size and runtime
• mp-300k: blocked on the legacy/patched surface by size/runtime/benchmark manifest gates