fix(build): auto-init submodules, auto-apply lwIP patch, fix corrupt patch

fix(build): auto-init submodules, auto-apply lwIP patch, fix corrupt patch

The build system now handles submodules and patches automatically so
a fresh 'git clone' + 'make' works without manual steps:

- Makefile: add LWIP_SENTINEL target that runs 'git submodule update
  --init --recursive' if lwIP sources are missing.
- Makefile: add .lwip_patched marker target that applies the lwIP
  volatile patch (patches/lwip-tcpip-volatile.patch) once after
  submodule init. Kernel build depends on this marker.
- Makefile: clean target now removes .lwip_patched marker.
- patches/lwip-tcpip-volatile.patch: regenerated with correct hunk
  header (@@ -55,9 +55,9 @@ instead of broken @@ -55,8 +55,8 @@)
  and repo-root-relative paths (third_party/lwip/...).
- user/doom/Makefile: fix error message to reference 'git submodule
  update --init' instead of 'git clone'. Check for actual header file
  instead of just directory existence.
- .gitignore: add .lwip_patched marker, remove temp hello/hello.c.