Fix actionable upstream issues

[SHIINASAMA]

Summary

This PR applies the actionable upstream findings that were grouped in the fork issues below. It is intentionally left open for review and is not merged.

• Refs Stabilize externally controlled fraction updates and animate them safely #1 — stabilize externally controlled fraction updates, clamp them to constraints, animate intentional external changes, and make default Split, HSplit, and VSplit FractionHolder state stable across SwiftUI view reconstruction.
• Refs Replace deprecated SwiftUI onChange overloads without dropping platform support #2 — replace deprecated one-argument onChange observation in the library, demo, and README without raising deployment targets.
• Refs Make the splitter cursor fix platform- and deployment-safe #3 — use native columnResizeCursor/rowResizeCursor on macOS 15 and later, retain a deployment-safe resizeLeftRight/resizeUpDown fallback for macOS 12–14, keep the existing Catalyst fallback, and prevent the cursor overlay from intercepting drag gestures.
• Refs Add direction-aware hide and show transitions for split sides and the splitter #4 — add direction-aware horizontal/vertical hide and show transitions for both split sides and the splitter, while leaving animation choice to the caller.

Upstream context:

• upstream #29, #36, #44
• upstream #41
• upstream PR #42
• upstream #48

Review follow-up

• Split now owns its default FractionHolder with @StateObject; .fraction(FractionHolder) keeps an explicit caller-owned holder separate and observed.
• All fluent modifiers now carry an uninstalled fractionSeed instead of reading a wrapped @StateObject; only body reads the installed object when composing the child Split.
• Explicit holders no longer seed any StateObject, so replacing a caller-owned holder during a mounted View rebuild does not retain the old holder in SwiftUI state storage.
• Removed the unconditional .animation(.default, value: hide.side?.rawValue) so callers control whether hide/show changes animate and which transaction animation is used.
• Added a deployment- and SDK-safe runtime resolver for the modern macOS 15 cursor selectors, with the macOS 12–14 fallback preserved.
• Added regression coverage for Split holder identity, forced hide/show animation, and modern/fallback cursor availability. The onChange source check now targets the deprecated one-argument closure shape instead of matching every overload.
• Added a mounted NSHostingView rebuild regression test that verifies a replaced caller-owned holder is released and no pre-install StateObject access warning occurs.

Implementation notes

• Public Split/HSplit/VSplit APIs are unchanged.
• Fraction updates use publisher-backed observation and avoid re-publishing an unchanged clamped value.
• Explicit FractionHolder inputs remain caller-owned across all Split modifiers.
• macOS cursor handling uses update-safe AppKit cursor rects; Catalyst retains the existing hover fallback.

Verification

• swift test --scratch-path /tmp/SplitView-review-full-tests — 14 tests, 0 failures
• swift test --scratch-path /tmp/SplitView-review-lifecycle-green --filter 'ObservationTests|StateObjectLifecycleTests' — 4 tests, 0 failures
• swift build --scratch-path /tmp/SplitView-review-macos — passed
• swift build --sdk "$(xcrun --sdk iphoneos --show-sdk-path)" --triple arm64-apple-ios15.0 --scratch-path /tmp/SplitView-review-iphoneos — passed
• swift build --sdk "$(xcrun --sdk macosx --show-sdk-path)" --triple arm64-apple-ios15.0-macabi --scratch-path /tmp/SplitView-review-catalyst — passed
• Demo xcodebuild builds for macOS, iOS, and Mac Catalyst — all exit 0; the Demo currently resolves the upstream Package URL, so these validate the example project rather than directly integrating the local fork checkout
• git diff --check — passed

No comments or messages were posted to the upstream repository.