fix(debug): target the traced thread and keep the kill from ever blocking

The Go runtime can migrate the debuggee's target-mode goroutine off the
process leader before PTRACE_TRACEME, which left the trace relation on a
thread the session never addressed: its stops starved the waits on the
leader, and a kill sequence that resumed nothing and then blocked in
Wait4 hung the whole package.  The debuggee now reports the traced thread
in the launch handshake and parks with a thread-directed stop, every
ptrace request and wait addresses that thread, a SIGURG arriving on a
single-step resumes it as a single-step again instead of letting the
tracee run uncontrolled, a resume rejected with ESRCH lifts a group-stop
with SIGCONT and retries once, and Kill resumes, kills and reaps through
non-blocking waits so it returns for a tracee in any state.

Assisted-by: GLM 5.3

debug/target_linux.go

@@ -29,6 +29,18 @@ func mapRWX(code []byte) ([]byte, error) {
 	return mem, nil
 }
 
+// parkSelf stops the calling thread for the launch handshake.  The stop is
+// thread-directed on purpose: a process-directed SIGSTOP may be queued on
+// any of the debuggee's untraced runtime threads, and delivering it there
+// establishes a group-stop over the whole thread group, which leaves the
+// traced leader in a plain stop the tracer cannot resume (PTRACE_CONT fails
+// ESRCH) and the siblings stopped with nobody to continue them.  Directed
+// at the traced thread, the stop always surfaces as the signal-delivery-stop
+// the handshake consumes.
+func parkSelf() {
+	syscall.Tgkill(syscall.Getpid(), syscall.Gettid(), syscall.SIGSTOP)
+}
+
 // setupBuffers allocates buffers in the debuggee's memory.
 func setupBuffers(spec string, args []byte, tmpDir string) ([]byte, error) {
 	type bufSpec struct {