Introduce a renderer.flush scheduler to batch dirty/removed message

…line highlights

…and extmark allocs

- Wrap apply_pending and end_bulk_mode writes in eventignore='all' so
  render-markdown only fires from the explicit request_on_data_rendered
  call, not once per nvim_buf_set_lines during streaming or session load
- set_lines noautocmd opts removed; bulk caller sets eventignore directly
  via begin_update/end_update, keeping the API clean
- set_extmarks: skip deepcopy for marks with no start_col and no end_row
  offset needed (~99% of extmarks), eliminating 125k copies per session load
- add_vertical_border: build extmark_opts once per call instead of per line
- get_markdown_filetype: memoize vim.filetype.match results by filename
- accumulate_bulk_extmarks helper deduplicates 3x copy-pasted extmark loop
- Remove dead bulk_message_positions/bulk_part_positions tracking fields
- Remove unused write_formatted_data export and dead append_part_now bulk branch
- Hoist lines_equal/extmarks_equal out of format_message closure; replace
  vim.inspect comparison with vim.deep_equal

…nd simplify flush

Remove unused ctx.prev_line_count from the renderer context and reset.
Simplify M.flush() to only request data-rendered when changes were applied and
the renderer is not in bulk_mode. Extract window-scrolling into
M.scroll_win_to_bottom() and reuse it from post_flush().

tests: make replay deterministic and robust for bulk mode

Sort extmarks deterministically in test helpers, clear/reset UI and renderer
state during replay setup, and force synchronous completion (flush.end_bulk_mode())
in replay tests when ctx.bulk_mode is active so assertions run against final output.

- dialog: add support to extend vertical border to trailing blank via config.extend_border_to_trailing_blank
- renderer: wrap full-session rendering in bulk flush (begin/end) and delegate scroll-to-bottom to scroll module
- output_window: simplify visible-bottom-line calculation to use vim.fn.line('w$')
- ui: guard treesitter language registration with pcall to avoid errors in minimal builds