POSIX process launch hides exec failures and loses the actual error #211

POSIX process creation returns a process handle even when execvp fails. It also labels permission failures as command-not-found and loses errno. The caller cannot reliably distinguish a launch failure from a program that genuinely exits with code 127.

Regression coverage first: add test/regression/ProcessPosixStartError in the existing regression layout. Check that an explicit missing executable fails during creation. Extend coverage to a file without execute permission and to a real child that exits 127, which must remain a normal process result. Keep the args=null shell-command path separate from direct executable invocation.

Confirmed on Linux x86_64 with Haxe 4.3.3:

missing executable: constructor succeeded, status=127
stderr: Command not found : /no-such-libhxcpp-command
existing non-executable file: constructor succeeded, status=127
stderr: Command not found : /tmp/libhxcpp-process-audit/not-executable

In src/hx/libs/std/Process.cpp, the parent reports success as soon as fork succeeds. The child handles every execvp error by writing a prebuilt generic message and calling _exit(127). Pipe and fork failure paths also return null without preserving their reason.

Use a launch-error handshake that carries errno from the pre-exec child back to the parent, or an appropriate spawn API. Keep the child path async-signal-safe. Successful exec should close the handshake automatically, and failed launch cleanup must close all pipe ends and reap the child.

Related to #75, which addressed native Windows diagnostics. Haxe's cpp Process constructor currently discards caught native error details too, so exposing those details through sys.io.Process needs a standard-library change. NativeProcess should still retain the actual POSIX launch error.