docs: add Issue #1 final frame brief

[RollNuts]

Overview

• Add docs/briefs/final-frame-brief.md as the one-page final-frame target brief for Issue M1-T01 Write the final-frame visual brief #1.
• The brief now defines hero/enemy/floor/UI/VFX/camera specs, a 30-second capture sequence, abstract-quality reference criteria, anti-copy constraints, and adopt/reject checklist.

Closes #1

Notes for Veripsa Core review

• Please review this brief for anti-copy compliance and 30-second capture feasibility before engine slicing starts.

[RollNuts]

Veripsa Core request: please include this PR in merge-order planning with #10, #12, and #13. This PR defines the final-frame target and closes #1. Please flag if it should merge before/after the free asset shortlist or engine ownership plan.