fix: forward mouse drag events in buttonEventTracking mode

[gi11es]

Summary

Fixes #87

• mouseDragged only forwarded drag events when mouseMode was .anyEvent (via sendMotionEvent()), but tmux uses .buttonEventTracking (CSI ? 1002 h) which also requires drag event forwarding for pane resize and mouse text selection.
• Changed the guard condition from sendMotionEvent() to sendButtonTracking(), which returns true for both buttonEventTracking and anyEvent modes.

Reproduction

1. Open a tmux session inside SwiftTerm
2. Try to resize a pane by dragging the border
3. Before fix: nothing happens — drag events are silently dropped
4. After fix: pane resizes correctly, mouse selection works

Test plan

• Added MouseTrackingTests with 14 tests covering:
  • MouseMode.sendButtonTracking() returns correct values for all 5 enum cases
  • MouseMode.sendMotionEvent() returns correct values for all 5 enum cases
  • sendButtonTracking() is a superset of sendMotionEvent() (the invariant this fix relies on)
  • Integration: motion events produce SGR output when buttonEventTracking is active
  • Integration: motion events produce SGR output when anyEvent is active (regression guard)
  • Negative: off and vt200 modes do not pass the sendButtonTracking() check
  • SGR encoding format matches what tmux expects (<32;col;row M)
• All 14 tests pass via swift test --filter MouseTrackingTests

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