docs: full POSIX/Unix audit + fix git-clone breakage with submodules

docs: full POSIX/Unix audit + fix git-clone breakage with submodules

- Add docs/FULL_POSIX_AUDIT.md: comprehensive analysis of POSIX gaps,
  build system issues, and porting requirements for Newlib/GCC/Binutils/
  Bash/Busybox
- Convert lwIP and doomgeneric from untracked nested repos to proper
  git submodules (.gitmodules) so git clone --recursive works
- Update .gitignore: remove third_party/ and user/doom/doomgeneric
  exclusions now that they are tracked as submodules

Build system was broken after git clone: lwIP (required) and doomgeneric
(optional) directories were empty because they were gitignored nested
repos with no .gitmodules. Now users can:
  git clone --recursive <url>
  make CROSS=1 && make iso