feat(tui-go): scroll tall modals instead of clipping them off-screen

The ? help and S stats modals rendered at fixed height; compositeOverlay
drops any rows past the screen, so on a 24-30 row terminal their bottom
sections — including the close hint — vanished with no way to reach them
(and the backdrop border corrupted). Recent help additions worsened it.

- Add a shared scrollable-modal renderer: helpBody()/statsBody() produce
  discrete lines, renderScrollModal() windows them by m.modalScroll (clamped
  to the viewport via modalBodyRows/scrollableModalMax) and pins the hint, so
  the box never exceeds the screen. A "(off-end/total)" indicator shows in the
  title when scrolled. Keys: ↑↓/jk line, PgUp/PgDn page, ^u/^d half, g/G ends;
  any other key still dismisses help. modal_scroll_test.go (2): clamp + no-op.
- Page the artifacts viewer (v) too: PgUp/PgDn were one line at a time — now a
  full body, with ^u/^d half and g/G ends via scrollArtifact().

Verified via cmd/preview: help/stats close hints now survive at h=20-28 (were
clipped below ~32/40) and show the scroll indicator; full at tall heights.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/tui-go/internal/app/model.go

@@ -271,6 +271,7 @@ type Model struct {
 	overlay          OverlayKind
 	overlayEventIdx  int
 	diffScrollOffset int
+	modalScroll      int // body scroll for tall fixed-content modals (help, stats)
 	eventStripShown  bool
 
 	// artifact viewer (OverlayArtifacts) — populated by the artifact.list response