docs: reconcile V2_RESOLUTION.md's open questions against shipped v2

[collinsezedike]

Summary

V2_RESOLUTION.md's status banner still read "Proposed; design-only and not implemented," but contracts/tholos-v2 has since fully implemented the mechanism it describes. This checks each of the document's 9 open design-review questions against the shipped code and updates the document to reflect what actually happened.

Closes #115

Changes

• Status banner updated to "Implemented," pointing to CONTRACT_V2.md as the authoritative interface reference and framing this document as retained design rationale.
• Each of the 9 "Questions for design review" now states whether the shipped code answers it, with file:line citations into contracts/tholos-v2/src/lib.rs. 6 of 9 are answered by explicit implementation choices; 3 (non-refundable dispute fee, TTL-restoration funding, market/question identifier) remain genuinely open and are called out as such rather than assumed resolved.
• Three other now-stale "must receive maintainer approval" / "blocking security review" sentences in the body, which the closing questions section already contradicted, updated to point at the actual outcome.
• "Future implementation work" checklist updated to show each item's tracking issue and completion status, including that the independent security audit remains undone and tracked privately, per SECURITY.md.

No code changes.