Add a seven-page visual PDF that explains how the Sudoku solver parses input,
builds row/column/block constraints, stores candidates as bit masks, applies
naked and hidden singles, and orders recursive search branches.
The guide is authored as plain HTML/CSS/SVG so the diagrams stay reviewable and
can be regenerated without a separate design tool. The README now links to the
PDF artifact for quick discovery.
Test Plan:
- cargo test
- pdfinfo docs/solver-visual-guide.pdf
- Raster-inspected exported PDF pages with pdftoppm
Refs: none
