tools: stream UVM trace via trace_pipe in the guest pin scanner (kill the second-client stall race)

The guest scanner's TraceReader tailed the last 256 KB of the trace ring
every 0.3 s. A one-shot uvmt sem= event could age out of that window
before the next poll (the client's spin floods the ring), so a second
client in the same VM was never discovered: semfix re-pinned the previous
client's freed sem while the new client's push page stayed untranslated
(host nomap, drain held, guest spins forever).

- guest_pin_stream.py: consume /sys/kernel/tracing/trace_pipe instead
  (each event delivered exactly once; no ring-window race), with
  reconnect on read error. The old ring-tail read is kept only as
  semfix's fallback.
- clear the trace ring on every client switch so the fallback cannot
  re-learn the previous client's stale events.

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