[GrantFox][High] Prove market settlement conservation

Summary

Prove market settlement conservation

Why this matters

Settlement must distribute the exact available value without double payment or stranded balances.

Scope

Define conservation equations and enforce them across winning, losing, void, and partial-liquidity outcomes.

Acceptance criteria

• Debits equal credits plus documented fees/remainders.
• Each market settles once.
• Void and partial outcomes conserve funds.
• Property tests cover minimum, maximum, and adversarial amounts.

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.

No implementation file, entry point, or named test path is provided. Begin by locating the market settlement logic and existing property-test coverage, then define the conservation equations and failure cases before adding tests for winning, losing, void, partial-liquidity, minimum, maximum, and adversarial outcomes; done means all acceptance criteria pass in repository CI.