Vim motion support

[KyawTheMonkey]

Summary

Adds opt-in vim-style keyboard navigation to all interactive views in TUIkit, alongside a complementary horizontal navigation modifier.

What changed

New public API — two composable modifiers:

// Vertical (up/down within a list, menu, or VStack)
.verticalNavigationStyle(.vim)
.verticalNavigationStyle(.vim, .arrowKey)   // both active

// Horizontal (Tab-cycling between sections/panels)
.horizontalNavigationStyle(.vim)
.horizontalNavigationStyle(.tab, .vim)      // both active

Both modifiers accept a variadic Set of styles, so each scheme is fully opt-in and can be combined freely. Neither changes the default behaviour. Existing apps get [.arrowKey] vertical and [.tab] horizontal with zero migration required.

Checklist

• Swift 6.0 compatible (no features requiring a newer compiler)
• Builds on macOS and Linux (swift build succeeds on both)
• All tests pass (swift test)
• No new SwiftLint warnings (swiftlint)
• Public APIs match SwiftUI signatures (parameter names, order, types)
• Modifiers propagate through the View hierarchy

Test Plan

35 new tests across 4 new suites in 2 existing test files

Tests/TUIkitTests/ItemListHandlerTests.swift:

ItemListHandlerVimMotionTests (15 tests) — unit tests for every vim binding directly on ItemListHandler:

• j moves down and wraps at the end
• k moves up and wraps at the start
• g jumps to index 0; respects selectableIndices (jumps to first selectable)
• G jumps to last index; respects selectableIndices
• Ctrl+d moves max(1, viewportHeight/2) forward, clamps at last item, moves at least 1 step even when viewport height is 1
• Ctrl+u moves half page backward, clamps at first item
• Ctrl+f moves a full page forward
• Ctrl+b moves a full page backward
• ItemListHandlerNavigationStyleGatingTests (5 tests) — verifies the opt-in/opt-out gating:
  • Default [.arrowKey]: j/k return false and don't move the cursor
  • [.vim] only: .down/.home/.pageDown return false; vim keys work
  • [.arrowKey, .vim]: both sets work simultaneously
  • [] empty set: all navigation keys return false

Tests/TUIkitTests/FocusManagerTests.swift:

FocusManagerVerticalVimTests (5 tests) — covers j/k in FocusManager.dispatchKeyEvent:

• Default style: j/k not consumed by the fallback
• [.vim]: j calls focusNextInSection, k calls focusPreviousInSection
• [.vim] only: .down arrow not consumed by the fallback
• [.arrowKey, .vim]: both key sets drive section navigation
• beginRenderPass() resets verticalNavigationStyles back to [.arrowKey]

FocusManagerHorizontalVimTests (8 tests) — covers h/l and Tab gating:

• Default [.tab]: h/l not consumed, Tab cycles focus
• [.vim] only: l → focusNext, h → focusPrevious; Tab returns false
• [.tab] only: l/h not consumed
• [.tab, .vim]: Tab, l, and h all cycle focus correctly
• beginRenderPass() resets horizontalNavigationStyles back to [.tab]