docs: update all documentation for DOOM port, euid/egid, /dev/fb0, /dev/kbd, fd-backe...

docs: update all documentation for DOOM port, euid/egid, /dev/fb0, /dev/kbd, fd-backed mmap

README.md:
- Added DOOM port (/bin/doom.elf), /dev/fb0, /dev/kbd to features
- Updated permissions: euid/egid + VFS enforcement on open()
- Updated mmap: now includes fd-backed mappings
- Updated guard pages: kernel stacks at 0xC8000000
- Updated ulibc: added all new headers (stdlib.h, ctype.h, sys/mman.h, etc.)
- Updated POSIX score: 90% → 93%
- Removed file-backed mmap from remaining work (now implemented)
- Added user/doom/ to directory structure

BUILD_GUIDE.md:
- Added Section 3: Building DOOM (setup, build, run instructions)
- Updated ulibc description with all new headers
- Added doom.elf to initrd listing

docs/POSIX_ROADMAP.md:
- Added geteuid/getegid/seteuid/setegid syscalls (all [x])
- Updated permissions entry: euid/egid + VFS enforcement
- Updated devfs: added /dev/fb0, /dev/kbd
- Updated mmap: fd-backed mappings
- Added /dev/kbd and /bin/doom.elf entries
- Added 8 additional features (tasks 32-39) to progress section
- Removed file-backed mmap from remaining gaps (now done)
- Updated ulibc header list

docs/AUDIT_REPORT.md:
- Updated 4.3 (guard pages): kernel stacks now fixed too
- Updated summary table: 4.3 now FIXED (user + kernel)
- Updated fix summary: 2 MODERATE fixed, 6 remaining

docs/SUPPLEMENTARY_ANALYSIS.md:
- Updated POSIX score: 90% → 93%
- Updated VMM summary: fd-backed mmap, kernel guard pages
- Updated process model: full euid/egid syscall list
- Updated memory management: fd-backed mmap, kernel guard pages
- Updated security: VFS permission enforcement
- Updated userland: all new ulibc headers + DOOM port
- Removed file-backed mmap from remaining gaps
- Updated remaining actions (file-backed mmap removed)
- Updated conclusion

docs/TESTING_PLAN.md:
- Added DOOM Smoke Test section describing integration test value