All 24 source files compile and link for DOS/4GW 32-bit target using
OpenWatcom V2 cross-compiler (wcc386 -bt=dos -mf -za99 -D__MSDOS__).
Platform portability fixes:
- hal_dos.c: use int386() instead of int86() for 32-bit DOS
- interp.c: mkdir() 1-arg on DOS, _dos_findfirst/findnext for FILES,
monotonic_time() portable wrapper for clock_gettime/clock()
- virmem.c: replace clock_gettime with portable time()/localtime()
- graphics.c: define M_PI, avoid non-constant aggregate initializers
- tui.c: guard sys/ioctl.h and sigaction, use signal() on DOS,
use HAL screen size instead of TIOCGWINSZ
Produces: gwbasic.exe (154KB LE executable, requires DOS4GW.EXE)
Linux build and all 72+14+69 tests unaffected.
platform/hal_dos.c: DOS HAL implementation using BIOS INT 10h for
screen/cursor, INT 16h for keyboard, direct character output via
BIOS write-char. Compile-time selection via __MSDOS__ define.
Linux HAL (hal_posix.c) unchanged -- full backward compatibility.
hal.h: add hal_dos_create() declaration under __MSDOS__ guard.
main.c, gwrt.c: select HAL at compile time via #ifdef __MSDOS__.
Makefile.dos: OpenWatcom wmake build file targeting DOS/4GW 32-bit.
Builds GWBASIC.EXE (interpreter), GWBASCOM.EXE (compiler),
GWRT.LIB (runtime library for compiled programs).
All 72 interpreter, 14 kernel, and 69 compiler tests continue to
pass on Linux (no regression).
