DATE$, TIME$, and TIMER now return real system date/time instead of
hardcoded values. Added directory and shell access statements with
proper GW-BASIC error codes (Path not found 76, File already exists 60).
Bump to v0.6.0, 52 tests.
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.
