Specify single-block limitation on puzzle 11 pattern

[dunnoconnor]

Added a blockquote note to book/src/puzzle_11/puzzle_11.md in the solution explanation section (after step 3, "Main window operation") that:

1. Explains the single-block assumption and why the current code is safe for this puzzle's config
2. Warns that copying the pattern into multi-block kernels would cause out-of-bounds shared memory reads
3. Shows the correct multi-block guard pattern using local_i checks with global memory fallbacks for halo elements

Closes Issue #220