docs(risk-register): strategic-review curation + MetricFrame integrity cluster

[Polichinel]

Curation of the technical risk register from a /review-rr strategic pass (no code change).

Changes

• Demote C-21 (stale EvaluationManager docstring) and C-23 (vestigial adapters/ package) to the tech-debt backlog (technical_debt_backlog.md §5) — Tier-4, no risk dimension.
• Re-tier C-22 (silent Pearson nan) 4 → 3 (reachable in normal use).
• Register the MetricFrame emit-artifact blind spots as new Cluster D: C-24 (cross-repo contract drift, Tier 2), C-25 (provenance version-label integrity, Tier 3), C-26 (float32 cast + unenforced schema_version, Tier 3).
• Document causal clusters A–D; sharpen C-05 trigger (action-based); flag the C-11/C-14 version-bump regression (reverted to 0.4.0 via PR fix wrong version number #20; tracked in Release views-evaluation (cut 0.5.0) shipping to_metric_frame — the evaluation-of-record emit #23).
• Structural: remove duplicated C-10 from Open Concerns, normalize separators, add a Demoted Concerns table (IDs retired, never reused).

Notes

• First commit of technical_risk_register.md — it was previously gitignored (reports/) and maintained locally; force-added here so the curated register is tracked. (The backlog was already tracked.)
• Docs-only. ruff clean; 281 tests pass. Triage verdict: CLEAN (counts, IDs, cross-refs all consistent).

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