Make the splitter cursor fix platform- and deployment-safe

[SHIINASAMA]

Context

This tracks upstream PR #42: Fix cursor issue when hovering splitter.

The PR replaces the current hover handling in Sources/SplitView/Splitter.swift:80-89 with a custom cursor modifier and NSCursor.columnResize or rowResize. The PR discussion notes that those cursor APIs are only available on macOS 15, while this package supports macOS 12. The proposed CursorModifier.swift also uses AppKit NSView types without a platform guard, so it cannot be copied into the cross-platform target unchanged.

Goal

Fix nested-splitter cursor reliability while preserving the package platform matrix and providing an appropriate fallback for older macOS and Catalyst versions.

Acceptance criteria

• macOS 15 uses the modern column or row resize cursor where available.
• macOS 12 through 14 retain a working fallback.
• iOS and Mac Catalyst builds do not compile AppKit-only code accidentally.
• Entering and exiting nested splitters does not leave a stale cursor pushed.
• Runtime behavior remains draggable even if cursor installation is unavailable.
• Add platform compile checks and a manual or demo verification path for nested splitters.

[SHIINASAMA]

Fixed by #5, which was merged on 2026-09-11 as ed01704. The implementation and tests for this issue are included in the merged PR.