BSAVE/BLOAD: save and load virtual memory blocks with 0xFD-header
binary format, operating on the current DEF SEG segment.
TUI color: tui_refresh emits ANSI SGR codes from cell attributes;
COLOR statement sets tui.current_attr when TUI is active.
Extended PEEK/POKE: CGA graphics framebuffer (interlaced layout) via
gfx_cga_peek/poke routed through virmem when gfx_active(); BIOS
keyboard shift flags (offset 0x17 bit 7 = insert mode).
Add bibliography to language reference. 64 tests, all passing.
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.
