Move ProjectedRun next to EventProjectionCache in run_state, since the
summary store both produces and consumes it, and give it a replay
constructor that owns the events-to-head derivation. load_projection now
returns RunNotFound directly instead of erasing it to None and having
callers rebuild it; load_run_projection is the single Option translation
point. install_in_memory_state reuses the existing From impl, and the
commit path passes its Arc through instead of unwrapping and
reallocating it.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
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