- Replace cursor+zc fold creation with :{from},{to}fold Ex commands
  — avoids cursor-triggered screen redraws (~90ms/fold in large buffers)
- Switch to foldmethod=manual and stay there — prevents foldexpr
  recalculation on every buffer line (~4.6s on 87K lines)
- Lazy-render only viewport-sized message count on initial load
- Load more messages on scroll-to-top via WinScrolled autocmd
- Fix lazy_render_count being cleared by M.reset() — read before reset,
  persist back to ctx after determining limit
- Debounce WinScrolled load_more callback (150ms) to prevent rapid
  re-renders during fast scrolling
- Remove debug vim.notify logging from render paths

Performance: ~36s → ~10ms render time on a 2786-msg/87K-line session.

Closes sudo-tee#392

- base_picker: add preview='custom' + preview_fn type, Telescope
  buffer_previewer implementation for custom content preview
- output_window: extract apply_extmarks(bufnr, extmarks, line_offset?)
  as reusable public function; set_extmarks delegates to it
- api_client: list_messages(id, directory, opts?) accepts opts table
  for query parameters (e.g. limit)
- session: get_messages(session, opts?) passes opts through

Session picker now shows a rich preview when selecting a session:
- normalize_message_order: handle API descending order
- filter_preview_messages: show first user msg + last assistant reply
- format_messages: reuse existing formatter for consistent rendering
  (pcall per-part, no get_child_parts, no previous_message)
- render_preview_buffer: direct nvim API with window-local folds
- preview_fn: synchronous load via Promise:wait() (~20-30ms)

Only Telescope backend supported in this commit.

- Unify custom preview rendering behind a backend-neutral preview target so
  session previews work consistently with Telescope, fzf-lua, and Snacks.

- Normalize picker plugin aliases such as fzf-lua, telescope.nvim, and
  snacks.nvim to their internal backend names, and add regression coverage for
  custom preview behavior across the picker integrations.