perf(events): skip redundant writer dup probe when caller pre-checked

[Thunder-Blaze]

Issue

• writeEvents() was calling lookupEventById() on every event even when WriterPipeline had just run the same lookup in the same thread, so import/sync/router paid two B-tree seeks per event

Description

• WriterPipeline (used by strfry import / sync / router) already probes lookupEventById under a read-only txn right before opening its txn_rw and calling writeEvents(). The same probe then runs again inside writeEvents for every event. Consolidates to one probe on that path.

• Adds bool preChecked on EventToWrite. When set, writeEvents() skips lookupEventById but still runs the unconditional intra-batch guard (ev.id() == evs[i-1].id()). WriterPipeline sets the flag right after its pre-filter.

• RelayIngester → RelayWriter is intentionally not changed: the ingester's ro-txn is a batch-start snapshot and with multiple ingester threads its probe can be stale by the time the writer processes the message; skipping the writer probe there would race and double-insert.