Make the floating shelf an optional presentation style

[brzvsk]

What this changes

Adds an Edge Attached / Floating shelf setting, defaulting to the existing attached style. Floating adds 12-point side/bottom gaps, four rounded corners and a short slide transition. It keeps the nonactivating panel behavior and preserves Paste Stack and the existing close-completion safeguards.

The floating animation moves content inside a clipped, stationary window rather than moving the window below the display. This incorporates the fork's multi-monitor fix with one path that cannot spill onto a display below. Reduce Motion skips the floating slide. The shared glass helper retains Reduce Transparency and pre-macOS-26 fallback behavior.

Independent PR; no dependency on the appearance-choice PR.

How you tested it

• All 189 CopyCore tests passed, including attached/floating frames, standard/compact heights and offset display coordinates.

• Xcode Debug build passed; git diff --check passed.

• Reviewed close cancellation, deferred completions and bounded animation geometry. Physical multi-monitor animation and paste-in-another-app checks remain manual; the isolated demo build has no Accessibility grant.

• Every commit is signed off.

• Engine changes include tests.

• All three SPM pins are preserved.

[tarikbc]

Merged as cc1f552.

This one needed the most work, so flagging what I changed. It was written against the old shelfProDark flag, which #13 replaced with ShelfTheme, so I dropped the proDark parameters and threaded floatingShelf alongside the theme instead. ShelfPanelController.init now takes both.

I also removed the 'Always Use Dark Shelf' toggle this PR carried, because the Appearance picker from #13 supersedes it and shelfProDark no longer exists as a property. And I wrote a doc comment for floatingShelf to match the rest of that file.

The ShelfClippingView was a good catch. Keeping the window still and sliding only the clipped content is the right fix for a display sitting below the main one. I have not tested that case on hardware yet.

Closing because the rebase changed the SHAs. Thanks!