chore: add TODO for unbounded scrollOffset in diff view

The diffScrollOffset in NavUp/NavDown has no upper bound — it can
scroll past the last line. The ApprovalSurface clamps the viewport
with coerceIn/minOf, so it doesn't crash, but the offset keeps
growing. Marked with TODO(diff-scroll) in both handlers.

apps/tui/src/main/kotlin/com/correx/apps/tui/reducer/RootReducer.kt

@@ -194,6 +194,9 @@ object RootReducer {
             // Diff scrolling in APPROVAL mode when expanded.
             is Action.NavigateUp -> {
                 if (withBgReset.displayState == DisplayState.APPROVAL && withBgReset.diffExpanded) {
+                    // TODO(diff-scroll): cap scrollOffset at totalLines - PREVIEW_MAX_LINES to prevent
+                    // scrolling past the end of the diff. Needs total line count in state or a clamp
+                    // communicated from ApprovalSurface.
                     withBgReset.copy(diffScrollOffset = maxOf(0, withBgReset.diffScrollOffset - 1))
                 } else if (withBgReset.displayState == DisplayState.IN_SESSION && prevInputMode == InputMode.ROUTER) {
                     val selectedId = withBgReset.sessions.selectedId
@@ -235,6 +238,8 @@ object RootReducer {
             // Diff scrolling in APPROVAL mode when expanded.
             is Action.NavigateDown -> {
                 if (withBgReset.displayState == DisplayState.APPROVAL && withBgReset.diffExpanded) {
+                    // TODO(diff-scroll): cap scrollOffset at totalLines - PREVIEW_MAX_LINES; same issue
+                    // as NavigateUp above — the reducer doesn't know the diff length.
                     withBgReset.copy(diffScrollOffset = withBgReset.diffScrollOffset + 1)
                 } else if (withBgReset.displayState == DisplayState.IN_SESSION && prevInputMode == InputMode.ROUTER) {
                     val selectedId = withBgReset.sessions.selectedId