docs: add solver visual guide

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

README.md

@@ -32,6 +32,9 @@ cargo run -- sudoku-examples/hard.sudoku
 cargo run -- sudoku-examples/extreme.sudoku
 ```
 
+A visual explanation of the solver is available in
+[`docs/solver-visual-guide.pdf`](docs/solver-visual-guide.pdf).
+
 ## Project Decomposition
 
 - Command-line interface: `src/main.rs`