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/update.go

@@ -270,6 +270,7 @@ func (m Model) handleNormalKey(k tea.KeyMsg) (tea.Model, tea.Cmd) {
 		m.enterInsert(ModeRouter)
 	case "?":
 		m.overlay = OverlayHelp
+		m.modalScroll = 0
 	case "/":
 		m.enterInsert(ModeFilter)
 	case "j":
@@ -687,8 +688,29 @@ func (m Model) handleOverlayKey(k tea.KeyMsg) (tea.Model, tea.Cmd) {
 			}
 		}
 	case OverlayHelp:
-		// Any key dismisses the cheat-sheet (esc handled above).
-		m.overlay = OverlayNone
+		// The cheat-sheet scrolls when it's taller than the screen; any non-scroll key
+		// dismisses it (esc handled above), keeping the quick-glance feel.
+		page := m.modalBodyRows()
+		switch {
+		case k.Type == tea.KeyUp || runeIs(k, "k"):
+			m.scrollModal(-1)
+		case k.Type == tea.KeyDown || runeIs(k, "j"):
+			m.scrollModal(1)
+		case k.Type == tea.KeyPgUp:
+			m.scrollModal(-page)
+		case k.Type == tea.KeyPgDown:
+			m.scrollModal(page)
+		case k.Type == tea.KeyCtrlU:
+			m.scrollModal(-page / 2)
+		case k.Type == tea.KeyCtrlD:
+			m.scrollModal(page / 2)
+		case runeIs(k, "g"):
+			m.modalScroll = 0
+		case runeIs(k, "G"):
+			m.modalScroll = m.scrollableModalMax(len(m.activeModalBody()))
+		default:
+			m.overlay = OverlayNone
+		}
 	case OverlayToolPalette:
 		if runeIs(k, "t") {
 			m.overlay = OverlayNone
@@ -706,14 +728,17 @@ func (m Model) handleOverlayKey(k tea.KeyMsg) (tea.Model, tea.Cmd) {
 				m.artifactScroll = 0
 			}
 		case k.Type == tea.KeyPgUp:
-			if m.artifactScroll > 0 {
-				m.artifactScroll--
-			}
+			m.scrollArtifact(-m.artifactContentBodyH())
 		case k.Type == tea.KeyPgDown:
-			contentW := m.modalWidth() - 6
-			if m.artifactScroll < m.artifactContentMaxScroll(contentW) {
-				m.artifactScroll++
-			}
+			m.scrollArtifact(m.artifactContentBodyH())
+		case k.Type == tea.KeyCtrlU:
+			m.scrollArtifact(-m.artifactContentBodyH() / 2)
+		case k.Type == tea.KeyCtrlD:
+			m.scrollArtifact(m.artifactContentBodyH() / 2)
+		case runeIs(k, "g"):
+			m.artifactScroll = 0
+		case runeIs(k, "G"):
+			m.artifactScroll = m.artifactContentMaxScroll(m.modalWidth() - 6)
 		case runeIs(k, "v"):
 			m.overlay = OverlayNone
 		}
@@ -739,7 +764,25 @@ func (m Model) handleOverlayKey(k tea.KeyMsg) (tea.Model, tea.Cmd) {
 			m.overlay = OverlayNone
 		}
 	case OverlayStats:
-		if runeIs(k, "S") {
+		page := m.modalBodyRows()
+		switch {
+		case k.Type == tea.KeyUp || runeIs(k, "k"):
+			m.scrollModal(-1)
+		case k.Type == tea.KeyDown || runeIs(k, "j"):
+			m.scrollModal(1)
+		case k.Type == tea.KeyPgUp:
+			m.scrollModal(-page)
+		case k.Type == tea.KeyPgDown:
+			m.scrollModal(page)
+		case k.Type == tea.KeyCtrlU:
+			m.scrollModal(-page / 2)
+		case k.Type == tea.KeyCtrlD:
+			m.scrollModal(page / 2)
+		case runeIs(k, "g"):
+			m.modalScroll = 0
+		case runeIs(k, "G"):
+			m.modalScroll = m.scrollableModalMax(len(m.activeModalBody()))
+		case runeIs(k, "S"):
 			m.overlay = OverlayNone
 		}
 	case OverlayIdeas:
@@ -962,6 +1005,7 @@ func (m *Model) openStats() {
 		return
 	}
 	m.overlay = OverlayStats
+	m.modalScroll = 0
 	// Reuse a cached report only if it's for this session; otherwise show a loading state.
 	if m.statsFor != m.selectedID {
 		m.stats = nil
@@ -1082,6 +1126,7 @@ func (m Model) execPalette(id string) (tea.Model, tea.Cmd) {
 		m.toggleInlineActions()
 	case "help":
 		m.overlay = OverlayHelp
+		m.modalScroll = 0
 	case "mode":
 		m.cycleChatMode()
 	case "cancel":