feat(sync): shadow root mode (research) - #548

What this does

sync can now run without touching the site's html, body or panel styles at all. flex and grid bodies, body padding, sticky footers, multi-child panels and iOS-style scroll locks all behave like the plain page instead of breaking. it's opt-in behind shadow: true so you can compare it side by side with the current approach.

• sync attaches a shadow root to body (or the panel) and builds track > content > layout > <slot> there, the site's children get slotted in untouched
• the layout box copies body's display, alignment and min-height, gap and grid tracks are inherited, so a flex/grid body keeps laying out its children
• anything above the track (body padding, margins) scrolls natively and gets subtracted as an offset, re-read when html/body style or class changes (scroll locks)
• destroy() can't remove a shadow root, it leaves a bare <slot> that renders like nothing is there, and reuses it on the next construct (StrictMode, HMR)
• playground: /sync?shadow and /sync-nested?shadow, plus nav links

Checked (desktop chrome, positions compared against the same page without sync)

mirroring, scrollTo, scrollIntoView, focus, hash anchors, flex column body, grid body, padded body, sticky footer, position: fixed; top: -scrollY scroll lock, nested panel with 2 children, scroll-timeline via body::part(content). default mode re-checked, unchanged.

Not checked / known tradeoffs

• real iOS touch and rubber-band, safari, firefox, find-in-page through the slot
• body padding-top is no longer synced (it was inside body before), it moves natively like a plain page
• body { scroll-timeline } has to become body::part(content)
• offsetParent skips the shadow boxes, so offsetTop-chain math (the playground ghosts, older parallax libs) might be off
• only watches style/class, so locks like react-remove-scroll (data-scroll-locked + a style tag) aren't picked up yet
• hosts that can't take a shadow root (ul, td, custom elements with their own) would need the current path as fallback