feat: DOOM port — doomgeneric AdrOS adapter + remaining ulibc extensions

feat: DOOM port — doomgeneric AdrOS adapter + remaining ulibc extensions

Added user/doom/ with the AdrOS platform adapter for doomgeneric:
- doomgeneric_adros.c: implements DG_Init (fb0 mmap + kbd open),
  DG_DrawFrame (nearest-neighbor scale to framebuffer),
  DG_GetKey (PS/2 scancode → DOOM keycode mapping),
  DG_GetTicksMs (clock_gettime), DG_SleepMs (nanosleep)
- Makefile: builds doom.elf from doomgeneric source + adapter
- README.md: setup instructions

Additional ulibc functions for DOOM engine compatibility:
- ctype.h: isdigit, isspace, isalpha, toupper, tolower, etc.
- stdlib: strtol (base 8/10/16 + auto-detect)
- string: strncat, strdup, strcasecmp, strncasecmp, strstr,
  memchr, strtok
- stdio: fseek, ftell, rewind, sprintf, sscanf, remove

To build DOOM:
  cd user/doom && git clone https://github.com/ozkl/doomgeneric.git && make