Add replacement-readiness dashboard gates for Microplex record-count tiers

[MaxGhenis]

Parent plan: #16

Goal: make Microplex replacement readiness visible from one durable dashboard artifact instead of ad hoc logs.

Deliverables:

• Define a stable loss-result JSON schema.
• Index mp-300k, mp-3m-rich, mp-3m-fast, and future mp-30m candidate artifacts.
• Add dashboard cells for target loss, protected target families, high-salience aggregates, microsim benchmarks, H5 compatibility, runtime, artifact size, record counts, positive weights, and ESS.
• Publish a current-candidate artifact path and score bundle.

Exit criterion: the dashboard can answer whether each Microplex record-count tier is passing, failing, or unmeasured for each release gate.

[MaxGhenis]

Umbrella discussion: #16