docs: add hard Sudoku example

Transcribe the hard 9x9 Sudoku puzzle from the provided image into the
plain-text example format accepted by the solver. The new example gives users a
ready-made input file for manual CLI checks.

Document the example directory in the README project decomposition and include a
command showing how to run the solver against this puzzle.

Test Plan:
- cargo run -- sudoku-examples/hard.sudoku

Refs: none

README.md

@@ -25,12 +25,20 @@ The grid width must equal the grid height, and the size must have square
 blocks. That supports layouts such as 4x4 with 2x2 blocks, 9x9 with 3x3
 blocks, and 16x16 with 4x4 blocks.
 
+Example puzzles live in `sudoku-examples/`. For instance:
+
+```sh
+cargo run -- sudoku-examples/hard.sudoku
+```
+
 ## Project Decomposition
 
 - Command-line interface: `src/main.rs`
   - Parses the `./sudoku-ai <sudoku_file>` argument shape.
   - Reads puzzle text from disk.
   - Prints the solved grid or a user-facing error.
+- Example puzzles: `sudoku-examples/`
+  - Stores reusable plain-text Sudoku inputs for manual solver checks.
 - Solver library: `src/lib.rs`
   - Plain-text parser
     - Ignores blank lines.