session.events.since reports truncated=true when the ring evicted events
between the client's watermark and its oldest retained seq. The client
ignored the flag and dispatched the retained tail as a gap-free replay,
painting responses missing their prefix, tool state or terminal event.
On truncation, skip the window up to the server's head without
dispatching it. The session's replay barrier still resolves true, so the
reconnect backstop's history read (#94779) resyncs from the stored
transcript; parked live frames newer than the head still dispatch.
Covers the client half of #100122; the server-side session-eviction
seq reset is #124587.
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