feat(userland): ported the CPython 3.12 interpreter

[FlareCoding]

Note

Medium Risk
Touches kernel syscalls, handle flags, VMA stack size, and PTY ioctls alongside a large new cross-build path; regressions could affect all userland static links and TTY/shell apps, not only Python.

Overview
Adds a static CPython 3.12.8 userland app that cross-builds against the musl sysroot, applies Stellux-specific configure/patches, trims the stdlib, and stages /usr + bin/python3 into a per-arch rootfs overlay copied into initrd (with initrd/usr/ gitignored).

Build/CI now builds and caches compiler-rt builtins per arch (make compiler-rt, libclang_rt.builtins-*) so static links (especially Python on aarch64) do not depend on host cross builtins; toolchain-check, help, and userland/mk/toolchain.mk prefer sysroot builtins.

Kernel/userland fixes to support Python and richer TTY behavior: O_CLOEXEC on open with fcntl F_GETFD/F_SETFD (CLOEXEC stored in handle flags, masked from F_GETFL/F_SETFL); user stack grown to 512 KiB; PTY slave Linux termios/winsize ioctls (TCGETS/TCSETS, TIOCGWINSZ) plus Stellux raw/cooked ioctl IDs moved to 0x7301/0x7302 (user apps updated). CI kernel-unit-tests workflow caches/builds compiler-rt like libc++.

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