#595 Stage 2a — render_content: paint sidebar panel content

[JDonaghy]

Part of epic #595, milestone #9 TUI ShellApp Migration. Follow-on to
#601 (Stage 2), scoped out during its planning (see coord context vimcode 601 / the #601 discussion). Depends on #601.

Goal

Paint the active sidebar panel's content (explorer tree, git/source
control, search, debug, settings, extensions, AI panels) from
TuiShellApp::render_content, into AppShellLayout::sidebar_content_bounds.
quadraui's AppShell::render already paints the generic sidebar chrome
(activity bar + header) before render_content runs — this issue is only
the app-specific panel body content, mirrored from
panels.rs::render_sidebar's dispatch.

Scope / known gap

panels.rs::render_sidebar and several of its panel sub-renderers (e.g.
the explorer's background-fill loop) do raw frame.buffer_mut() writes
with no Backend::draw_* trait equivalent — the same class of gap
#601 hit for editor-window separators (render_content cannot reach a raw
Frame/Buffer; only &mut dyn quadraui::Backend trait calls work there,
see #601's shell_app.rs module doc for why). Each panel needs auditing:

• Panels that are already trait-pure (explorer's
  engine.explorer_tree.borrow().render(backend, rect), settings via
  Backend::draw_form) should port directly — generalize their helper
  signatures from concrete &mut TuiBackend to &mut dyn quadraui::Backend,
  same technique #595 Stage 2 — TuiShellApp::render_content paints for real through &mut dyn Backend #601 used for render_tab_bar/draw_breadcrumb_bar/
  render_window_status_line.
• Panels doing raw background fills need either an existing quadraui
  primitive that can stand in (check Backend::draw_status_bar with a
  blank solid segment — AppShell::render's own divider already uses this
  trick) or an explicitly documented, deferred gap. Do not guess a new
  quadraui primitive without checking first.

Acceptance

• driver_with_shell screen_contains assertion proving at least one
  sidebar panel's content renders via the ShellApp path (explorer is the
  default active panel — start there).
• cargo test --no-default-features green.
• Any panel left unported must be called out explicitly in
  shell_app.rs's module doc, not silently skipped.

Dependencies

Depends on #601. Blocks #605 (Stage 6 cutover) — cutover cannot ship with
sidebar content unpainted; the epic's own "no change to what gets
rendered" non-goal would be violated.