Refactor montage.jl: extract shared helpers, thin wrappers to reduce recompilations (#1929)

- Extract _montage_setup_frame!, _montage_divlines!, _montage_panels_size_str,
  _montage_adjust_margins! as shared helpers used by all three montage methods.
- Add thin outer wrappers that convert kw... to Dict immediately, calling typed
  inner functions (_montage_images, _montage_cube, _montage_choro) to avoid
  kwargs-driven method explosion.
- Replace nothing defaults with concrete sentinels: "" for strings, (0,0) for
  grid, Int[] for indices, false for divlines.
- Add type annotations to helper signatures (Dict, String, Bool, Vector{Int}).
- montage(GI) individual-CPT path now uses _montage_adjust_margins! for smart
  margin spacing with titles and colorbars.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: joa-quim