Add directional snap gesture mode

[muukii]

Summary

• add a public UIKit-backed .directional gesture mode
• use the existing axis for dominant-axis admission and snap offset updates
• separate successful end from cancellation or failure and document the mode

Why

scrollViewInteroperable handles ScrollView edge handoff, but simple cross-axis ownership should not require that contract. A horizontal snap gesture inside a vertical scrolling surface can now reject vertical and equal-diagonal input before beginning.

Validation

• 13/13 package tests on iPhone 17 / iOS 27.0 Simulator
• TinycurveTests 37/37 (46 parameterized executions) in the integration worktree
• Tinycurve iOS and native macOS Debug builds
• production-linked touch harness: vertical and diagonal scrolling, qualifying swipe exactly once, short and reverse input, Todo, context menu, and RTL
• explicit UIKit cancellation injection was unavailable; terminal mapping is covered by package tests and static review