feat: add steps parameter to MouseMove for smooth drag gestures

Makes MouseMove accept an optional steps variadic parameter that maps
to Playwright's MouseMoveOptions.Steps, generating intermediate
mousemove events for human-like drag behavior.

Closes #81

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

promote_test.go

@@ -15,7 +15,7 @@ func (m mockInteractiveBrowser) URL() string                            { return
 func (m mockInteractiveBrowser) MouseClick(float64, float64, string) error { return nil }
 func (m mockInteractiveBrowser) MouseDown(float64, float64, string) error  { return nil }
 func (m mockInteractiveBrowser) MouseUp(float64, float64, string) error    { return nil }
-func (m mockInteractiveBrowser) MouseMove(float64, float64) error          { return nil }
+func (m mockInteractiveBrowser) MouseMove(float64, float64, ...int) error  { return nil }
 func (m mockInteractiveBrowser) MouseWheel(float64, float64) error      { return nil }
 func (m mockInteractiveBrowser) KeyboardType(string) error              { return nil }
 func (m mockInteractiveBrowser) KeyboardPress(string) error             { return nil }