Adds per-overlay pattern fills so factions and regions can be visually
distinguished when they overlap (issue #25). Each faction note and each
region note now carries a pattern, scale, and opacity in its
frontmatter, edited via the existing FactionEditor / GeoRegionEditor
modals (dropdown + sliders + live preview swatch).
Pattern set (10 total): solid (the existing flat fill, default),
diagonal stripes ↗ / ↖, crosshatch, polka dots, grid, zigzag,
triangles, scales, checker.
On screen: SVG <pattern> defs are deduped per (key,color,scale) and
referenced from each overlay path's fill. Stroke stays solid color so
blob boundaries remain crisp. Per-overlay opacity replaces the
hardcoded 0.45 group opacity.
PNG export: mirrored via Canvas patterns built from an offscreen tile
per (key,color,scale), cached for the render pass.
Backwards compatible — overlays without the new frontmatter keys fall
back to solid + 0.45 opacity, matching pre-1.3.0 behavior.
