Add GitHub Actions CI with automated build and test. Implement real
terminal I/O with raw mode (enable_raw/disable_raw, proper INKEY$
polling via VMIN=0/VTIME=0, INPUT$ function). Add Sixel graphics
engine with virtual framebuffer (SCREEN 1: 320x200, SCREEN 2:
640x200), Bresenham line drawing, midpoint circle, flood fill PAINT,
DRAW mini-language parser, and Sixel encoder with RLE. Replace all
graphics stubs with real implementations (PSET, LINE, CIRCLE, DRAW,
PAINT, COLOR, SCREEN, POINT). Fix AND/OR/XOR operator precedence
to be lower than relational operators. Add 13 classic test programs
(39 total). Bump version to 0.5.0.
GW-BASIC reimplementation in C11, using Microsoft's open-sourced 8088
assembly as the authoritative reference.
Tokenizer (CRUNCH/LIST), expression evaluator with operator precedence,
all math functions (SIN, COS, TAN, ATN, SQR, LOG, EXP, RND, etc.),
string functions (LEFT$, RIGHT$, MID$, CHR$, ASC, VAL, STR$, etc.),
PRINT statement with comma/semicolon zones, TAB(), SPC().
Handles integer/single/double types with correct promotion, D exponent
for double-precision literals, type suffixes (%, !, #), &H hex/&O octal
literals, MBF conversion routines, and GW-BASIC-compatible number
formatting.
Platform-independent via HAL vtable; POSIX backend included.
