Link narrower BuddyKit products to fix duplicate Pasteboard class

[p-linnane]

Running any VirtualBuddy binary that loads both VirtualCore and VirtualUI (including vctool and the app itself) prints duplicate-class warnings at launch:

╰─ vctool -h
objc[8870]: Class _TtC13BuddyPlatform10Pasteboard is implemented in both /Applications/VirtualBuddy.app/Contents/Frameworks/VirtualCore.framework/Versions/A/VirtualCore (0x1011c92f0) and /Applications/VirtualBuddy.app/Contents/Frameworks/VirtualUI.framework/Versions/A/VirtualUI (0x100f525c8). This may cause spurious casting failures and mysterious crashes. One of the duplicates must be removed or renamed.
objc[8870]: Class _TtC13BuddyPlatform10Pasteboard is implemented in both /Applications/VirtualBuddy.app/Contents/Frameworks/VirtualCore.framework/Versions/A/VirtualCore (0x1011c92f0) and /Applications/VirtualBuddy.app/Contents/MacOS/VirtualBuddy (0x10066fc28). This may cause spurious casting failures and mysterious crashes. One of the duplicates must be removed or renamed.
OVERVIEW: Tools for updating the VirtualBuddy software catalog.

The cause is that VirtualCore, VirtualUI, the app, and VirtualBuddyGuest all linked BuddyKit's umbrella product, which statically embeds BuddyPlatform (and BuddyKitObjC) into each binary, so the same classes get registered by more than one loaded image. This PR links only the product each target needs and drops a duplicate BuddyKit package reference.

A few call sites that used umbrella-only API were updated to match. This also drops BuddyKit's debug-only PreviewBugFix, which isn't referenced anywhere in the repo. After the change vctool -h runs clean and nm confirms BuddyPlatform.Pasteboard is present only in VirtualUI.framework.

[insidegui]

This is the thing I dislike the most about Swift Package Manager. It's supposed to catch this sort of thing and turn the products into dynamic libraries when multiple targets link against them, but apparently it doesn't work reliably 😞

This also drops BuddyKit's debug-only PreviewBugFix, which isn't referenced anywhere in the repo

PreviewBugFix is activated automatically by the runtime, it's never referenced explicitly. Have you tested SwiftUI previews to make sure that the bug it is meant to fix is no longer an issue in recent Xcode versions?

[p-linnane]

Good catch, you were right about PreviewBugFix being runtime-activated via +load. I must have gotten some wires crossed during testing. I'm using Xcode 27 beta.

I updated the split so VirtualUI links the BuddyKit umbrella product, while VirtualCore and the app still link only BuddyFoundation. That keeps BuddyKitObjC/PreviewBugFix loaded once without reintroducing the duplicate class warning.

I re-tested with a SwiftUI preview and saw Posting preview terminate request., so the PreviewBugFix path is still active. I also rechecked the built app: BuddyPlatform.Pasteboard and PreviewBugFix both only show up in VirtualUI.framework.