W-990: render the picker glass with public API only

[wr]

Fixes the see-through, clipped picker that light-mode users see on macOS 27.2 with 1.10.0 (#230).

Cause

1.10.0 copied NSPopover's frostier glass by setting the private NSGlassEffectView._variant to 20 (#214). macOS 27.2 renumbered those private variants, so 20 now renders a clear lens:

• the panel turns see-through, with rainbow fringing at the edges
• the outer emoji columns, the search field and the tab bar refract out of view
• the corners frost into wedges

On 27.0, variant 19 reproduces the reporter's screenshot exactly. The responds(to:) guard only covered the key disappearing, not its numbers changing meaning. Dark mode never set the variant, so it was unaffected.

Fix

Drop the private call. Light mode now uses the public .clear glass style under the existing 0.25 white veil. Dark mode stays on unveiled .regular glass, as before. This covers every panel built on PopoverGlass: the list picker, the Quick Access pill, the emoji browser and the GIF panel.

On 27.0, .clear with the veil measures the same as the shipped look (~247 over white, ~117 over black) and blurs the page behind it just as heavily. Standard glass with the veil reads greyer over white and turns into a light slab over dark pages.

Test plan

• scripts/run-tests.sh
• Verified on macOS 27.2.0: the shipped build reproduces the bug, and this branch renders correctly
• A stand-in panel on 27.0 and 27.2 compared 1.9.1, 1.10.0 and the fix over light and dark pages. On 27.2 the private style breaks and .clear holds.

Refs W-990

[linear-code]

W-990