libc standards reset and C89 math core
* Normalize libc headers to standards-first APIs and types; move syscall numbers into new <apheleia/syscall.h> and update kernel/userspace dispatch points accordingly
* Add missing baseline C headers and runtime pieces (locale C/POSIX support, setjmp, assert runtime, wide/char type stubs, stdio and dirent surface cleanup)
* Fix libc behavioral mismatches in ctype/string/qsort/printf+scanf/time paths and migrate libc_usr wrappers plus user tools to the new interfaces
* Replace arch-specific libc math plumbing with a single self-contained C89 math implementation and keep complex support explicitly disabled
* Clean up related build and userland integration paths (syscall numbering, sleep/nanosleep routing, account/proc helpers, and command-side API updates)