ToolSettings: the active tool + each tool's OWN remembered ToolConfig
(color/width), so pen→highlighter→pen restores the pen's last color/width
instead of bleeding the highlighter's. Immutable copy-on-write (withActive/
withColor/withWidth only touch the active tool); sensible defaults (black thin
pen, yellow fat highlighter, medium eraser). The toolbar holds + persists one.
Pure model; fully unit-tested (per-tool isolation, immutability, equality).
flutter analyze lib/editor clean; 204/204 tests (+6).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
