## Summary

Adds critical rendering constraints to the `hyperframes` skill discovered from eval analysis of 27 agent-generated compositions. These guardrails prevent agents from producing compositions that technically work but render poorly.

## What it fixes

| Rule Added | Eval Prompts Affected | Issue |
| --- | --- | --- |
| Ban `repeat: -1` | #20 loading-spinner (2.0/5) | Infinite timeline broke capture engine |
| Ban async timeline construction | #16 particle-logo (2.6/5) | Timeline empty at capture time |
| Min font size 16px (labels), 20px (body) | #7, #8, #13, #14, #15, #19 | Illegible text after encoding |
| Ban full-screen dark linear gradients | #3, #5, #10, #14 | H.264 color banding |
| `<link>` fonts over CSS `@import` | #7, #24 | Font loading race conditions |

## Changes

- **Rules section**: Added `repeat: -1` ban, async timeline ban, items 8-9 to "Never do" list
- **Typography section**: Expanded font size guidance with specific minimums per text role (headlines, body, labels)
- **New "Backgrounds and Color" section**: Guidance on avoiding gradient banding
- **Output Checklist**: 5 new items covering all new constraints

## Test plan

- [ ] Run eval with updated skill and compare avg quality scores
- [x] Skill renders correctly in `/hyperframes` invocation