Sixel graphics: pure-Python decoder extracts ESC P...ESC \ sequences from
the output stream, renders RGBA pixels, and encodes as PNG for inline
display in the notebook. No external dependencies (no PIL, no Ghostscript).
INPUT support: when gwbasic prints "? " (INPUT prompt), the kernel uses
the Jupyter stdin protocol (raw_input) to request input from the user and
feeds the response back to the subprocess.
Pygments lexer (basic_lexer.py): GW-BASIC syntax highlighting with line
numbers, keywords, builtins, string/number literals, and comments.
Registered as a Pygments entry point and referenced in kernel language_info.
Test suite expanded from 10 to 14 tests (Sixel decode, PNG encode, inline
graphics integration, lexer tokenization).
