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/stopinfo_linux.go

@@ -47,7 +47,7 @@ func (s *Session) StopInfo() (StopReason, uint64) {
 	_, _, errno := syscall.Syscall6(
 		syscall.SYS_PTRACE,
 		uintptr(syscall.PTRACE_GETSIGINFO),
-		uintptr(s.pid),
+		uintptr(s.tid),
 		0,
 		uintptr(unsafe.Pointer(&info)),
 		0, 0,