tui: migrate off the deprecated Bubble Tea mouse Type API (SA1019)

The mouse classification helpers checked both the current Button/Action pair and
the deprecated msg.Type / tea.Mouse* constants. Bubble Tea's parser always
populates Button+Action and only DERIVES Type from them, so the Type fallbacks
were pure redundancy (a left-button drag is Action==Motion, already covered by
mouseMotion). Drop them — clears all 11 SA1019 findings in internal/tui with no
behavior change. The left-drag test now asserts the Action==Motion path directly.

Gates: gofmt/vet/build(host+linux+windows)/test -race/staticcheck(SA1019 0)/
deadcode(no new) all pass.

internal/tui/mouse.go

@@ -170,28 +170,28 @@ func (m model) syncMouseCapture() (model, tea.Cmd) {
 	return m, tea.DisableMouse
 }
 
-// Bubble Tea's Type field is deprecated, but its parser still populates it for
-// compatibility cases such as left-button drag events. Keep these helpers
-// tolerant of both the current Button/Action pair and legacy Type values.
+// Mouse classification uses the current Button/Action pair only. Bubble Tea's
+// parser always populates Button+Action and merely derives the deprecated Type
+// field from them, so checking Type adds nothing — and a left-button drag is
+// Action==Motion (which mouseMotion already covers), not a press.
 func mouseLeftPress(msg tea.MouseMsg) bool {
-	return msg.Button == tea.MouseButtonLeft && msg.Action == tea.MouseActionPress ||
-		msg.Type == tea.MouseLeft && msg.Action == tea.MouseActionPress
+	return msg.Button == tea.MouseButtonLeft && msg.Action == tea.MouseActionPress
 }
 
 func mouseMotion(msg tea.MouseMsg) bool {
-	return msg.Action == tea.MouseActionMotion || msg.Type == tea.MouseMotion
+	return msg.Action == tea.MouseActionMotion
 }
 
 func mouseRelease(msg tea.MouseMsg) bool {
-	return msg.Action == tea.MouseActionRelease || msg.Type == tea.MouseRelease
+	return msg.Action == tea.MouseActionRelease
 }
 
 func mouseWheelUp(msg tea.MouseMsg) bool {
-	return msg.Button == tea.MouseButtonWheelUp || msg.Type == tea.MouseWheelUp
+	return msg.Button == tea.MouseButtonWheelUp
 }
 
 func mouseWheelDown(msg tea.MouseMsg) bool {
-	return msg.Button == tea.MouseButtonWheelDown || msg.Type == tea.MouseWheelDown
+	return msg.Button == tea.MouseButtonWheelDown
 }
 
 func (m model) mouseOverComposer(msg tea.MouseMsg) bool {

internal/tui/mouse_test.go

@@ -348,12 +348,11 @@ func TestTranscriptSelectionLeftDragDoesNotResetAnchor(t *testing.T) {
 		Y:      1,
 	})
 	m = updated.(model)
-	// Bubble Tea marks left-button drag motion as Type MouseLeft for backward
-	// compatibility; this must update the cursor without resetting the anchor.
+	// A left-button drag is Action==Motion with Button==Left; this must update the
+	// cursor without resetting the selection anchor.
 	updated, _ = m.Update(tea.MouseMsg{
 		Button: tea.MouseButtonLeft,
 		Action: tea.MouseActionMotion,
-		Type:   tea.MouseLeft,
 		X:      8,
 		Y:      1,
 	})