[GrantFox][High] Harden oracle quorum and freshness enforcement

Summary

Harden oracle quorum and freshness enforcement

Why this matters

Unsafe price data can produce an incorrect market outcome.

Scope

Validate feed count, deviation, timestamp age, sign, and canonical-price persistence before settlement.

Acceptance criteria

• Stale or invalid feeds cannot settle.
• Quorum boundaries are deterministic.
• Canonical price updates are atomic.
• Outage and recovery tests cover all failure paths.

Validation

Add regression coverage for existing behavior, failure modes, authorization boundaries, and compatibility. The implementation must pass the repository CI checks.

Non-goals

• Typo-only, formatting-only, or documentation-only changes.
• Unrelated refactors or dependency upgrades.
• Weakening existing security, authorization, CI, or production safeguards.

Contributor application

Before implementation, comment with relevant experience, a 1–4 bullet approach, and an estimate for opening the first draft PR. Wait for maintainer assignment before coding.

PR requirements

Use a feature branch, include Closes #<issue-number>, check every acceptance criterion, link criteria to code/tests, explain security and failure-mode considerations, and pass CI.

Reward-readiness

This is a substantive GrantFox campaign issue. Merge and CI success do not by themselves guarantee reward eligibility; final reward-readiness is determined by campaign review.

Before coding, comment with relevant experience, a 1–4 bullet approach, and an estimate, then wait for maintainer assignment. Inspect the existing oracle settlement and feed-validation behavior, and add regression coverage for stale or invalid feeds, quorum boundaries, atomic canonical-price updates, outages, recovery, authorization, and compatibility. Done means all acceptance criteria pass with the repository CI checks.