posix-kernel: bump wasm-posix-kernel submodule to rebased tip

Rebases the wasm-posix-kernel submodule branch
playground-cli-experimental-posix-kernel-with-artifacts onto current
origin/main (42 commits forward). The superseded "set 128 KiB
stack-size" commit was dropped — main already enforces a 4 MB stack via
-Wl,-z,stack-size=4194304.

Author: mho22