Make Tweakable

Inject a floating Tweaks panel into an existing HTML artifact. Panel lives in-artifact (no iframe), uses File System Access API for persistence (with copy-paste fallback).

Pattern

1. Wrap tweakable defaults in CSS custom properties with --tweak- prefix on :root
2. Add marker block <script id="__tweak_schema" type="application/json">{...}</script> with the defaults
3. Inject floating panel (<div id="tweaks-panel">) with controls bound to each key
4. Panel writes updates to pending.yaml via File System Access API (asks user to select target file on first use)
5. Shift+T toggles panel visibility
6. User invokes /apply-tweaks <html-path> or says "save tweaks" — Claude reads pending.yaml, edits HTML via Edit tool, clears pending

Phase 1 — Decide keys