#609: #595 Stage 2c — render_content: window/group dividers, tab-drag overlay, tab-hover tooltip - #629

…verlay, tab-hover tooltip (Stage 2c)

Ports the three raw-Buffer/Frame writes #601 explicitly scoped out of
TuiShellApp::render_content:

- Window/editor-group divider lines: render_separators (within-group
  :split/:vsplit) and a new group_divider_cells/render_group_dividers
  (between editor groups, split.dividers) now paint through
  Backend::draw_status_bar via a 1-cell "solid StatusBar segment"
  trick -- the same one AppShell::render's own generic divider uses,
  confirming no new quadraui primitive is needed. The #481 phantom-
  divider-beside-scrollbar guard, previously a frame.buffer_mut()
  read-back, is now a pure data computation over RenderedWindow
  geometry shared by draw_frame and render_content alike.
- Tab-drag ghost overlay (render_tab_drag_overlay): dropped its Frame
  parameter; the ghost label now paints via the same draw_status_bar
  trick. Reads TuiShellApp's tui_drag_source/tui_drag_cursor/
  tui_tab_drop_zone, which #602 already wires handle_mouse_event to
  populate.
- Tab-hover tooltip (render_tab_hover_tooltip): sources from the plain
  engine.tab_hover_tooltip field, same trick.

render_all_windows now always paints separators regardless of the
frame: Option parameter, since the underlying paint no longer needs a
raw Buffer.

Adds driver_with_shell coverage for all three (group divider is the
issue's required headless case; tab-drag ghost and tab-hover tooltip
are bonus coverage since both turned out to be reachable without a
real PTY/raw-mode sequence). Updates shell_app.rs's module doc to mark
#609 closed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

… window height

- group_divider_cells's left_has_scrollbar check now excludes the
  neighbor window's per-window status-line row, matching
  window_overflows_vertically's own text_rows accounting. Previously
  it spanned the window's full rect height, so an overflowing left
  window with a status line wrongly looked scrollbar-covered on its
  status-line row too, leaving a 1-row gap in the group divider right
  at the neighbor's status bar. Added a direct unit test
  (group_divider_cells_covers_neighbor_status_line_row) that
  regresses this exact scenario.
- Hoisted the repeated backend.set_theme(...) call out of
  render_separators's and render_group_dividers's per-cell divider
  loops (draw_rule_cell_themed/draw_rule_row_themed) instead of
  rebuilding the ~50-field quadraui::Theme on every divider cell via
  draw_rule_row/draw_rule_cell.
- Documented why every divider/rule/ghost/tooltip draw shares the
  literal WidgetId::new("tui:rule") — currently inert, but a landmine
  for a future maintainer wiring hover/press state through this helper.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>