feat(estimator): refuse a checkpoint as the CPU f64 estimator

[seonghobae]

Summary

ADR 0001/0014 remaining buyer-visible scientific-authority increment: a model checkpoint is not the CPU f64 estimator. No new migration (0008 stays unallocated; #113 remains the entity/project SQL vehicle).

• refuse_checkpoint_as_estimator fails closed when a model_checkpoint is treated as cpu_f64_estimator.
• accept_checkpoint_artifact requires identity, model-run provenance, and canonical lowercase hex SHA-256; success does not grant estimator authority.
• Recovery is the computed share of artifact roles that match known truth versus collapsing every artifact to the estimator.

Test plan

• RED: checkpoint_authority_contract failed to compile (E0432) before the module existed
• cargo test -p checkpoint_authority --offline (lib + 5 contract tests)
• cargo clippy -p checkpoint_authority --all-targets --offline -- -D warnings
• python3 scripts/check_docstrings.py / check_workspace_contract.py / validate_documentation.py
• production lines 26/26 and nightly branches 18/18
• cargo test --workspace --offline --lib --tests
• exact-head required Checks on this PR (queued; do not treat as passed)

Do not self-approve or merge until exact-head required checks and a qualifying independent review pass. Keep this PR a draft. Do not empty-commit #46.

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