feat: DOOM compiles and links — adapter, build system, ulibc compat headers

feat: DOOM compiles and links — adapter, build system, ulibc compat headers

doom.elf (450KB) now builds successfully from doomgeneric source
with the AdrOS platform adapter.

Build system:
- user/doom/Makefile: excludes platform-specific adapters (SDL,
  allegro, emscripten, xlib, win, soso, linuxvt) and links with
  ulibc + crt0
- doomgeneric_adros.c: added main() entry point calling
  doomgeneric_Create/doomgeneric_Tick

New ulibc compatibility headers for DOOM:
- strings.h (wrapper for string.h)
- inttypes.h (PRId32/PRIu32/PRIx32 format macros)
- math.h (fabs/fabsf inline stubs)
- fcntl.h (O_RDONLY, O_WRONLY, O_CREAT, etc.)
- assert.h (assert macro with printf+exit)
- sys/types.h (ssize_t, off_t, pid_t, etc.)
- sys/stat.h (struct stat, S_ISDIR/S_ISREG)

New ulibc functions:
- stdlib: atof, system (stub), strtol
- All 19/19 kernel tests pass