feat(whaleflow): add student replay promotion gate

[Hmbown]

Summary

• add serializable StudentReplay result/metrics/test records on TeacherCandidate artifacts
• add deterministic PromotionGate evaluation for score delta, required tests, policy violations, stale replay, and cost delta
• update mirrored changelogs for the partial WhaleFlow: StudentReplay and PromotionGate for validated lessons #2675 offline slice

Stewardship boundary

This partially addresses #2675. It intentionally does not add Arcee execution, live provider calls, TraceStore writes, cached-main overlay writes, or automatic promotion side effects. The gate returns an explicit decision from recorded replay evidence only.

Credits: thanks @AdityaVG13 for the WhaleFlow draft and cost-tracking direction that shaped this lane.

Verification

• cargo test -p codewhale-whaleflow --locked
• cargo clippy -p codewhale-whaleflow --all-targets --locked -- -D warnings
• cargo fmt --all -- --check
• ./scripts/release/check-versions.sh
• ./scripts/release/check-ohos-deps.sh
• git diff --check
• cargo check --workspace --all-features --locked

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