v0.6.1 — popover UA-chrome fixes + flip-aware tooltip arrow

Patch on top of v0.6.0 (which was tagged but never published to rubygems — this is the version to install; all v0.6.0 notes apply).

Fixes (#31)

• DropdownMenu / Select: open panels rendered with a thick ring — the [popover] UA defaults (border: solid, padding, Canvas background) showing through the unstyled outer wrapper. The wrappers now reset the UA chrome.
• All popover panels: closed-state hiding now survives author display values (:not(:popover-open) { display: none } guard). Previously a host class like display: grid on a panel defeated the UA's [popover] { display: none } and left it permanently open.

Improvement (#32)

• Tooltip: the notch now follows flip fallbacks — pure CSS. The arrow's center chases the trigger via anchor() and is clamped inside the bubble's box, so it always sits on the trigger-facing edge, whichever side the bubble actually renders on. One rule replaces the four per-side arrow rules; the v0.6.0 "known limitation" is gone.

Suite green (263 runs); verified in Chrome and WebKit 26.5 (trusted input) across the affected gallery pages, including the theme-builder page that surfaced the stuck-open panel.