Adds an optional afterExecutionId field to SearchRequest. When combined
with a non-null timeFrom, ClickHouseSearchIndex applies a strictly-after
tuple predicate (start_time > ts OR (start_time = ts AND execution_id > id))
so same-millisecond exchanges can be consumed exactly once across ticks.
When afterExecutionId is null, timeFrom keeps its existing >= semantics —
no behaviour change for any current caller.
Also adds the SearchRequest.withCursor(ts, id) wither. Threads the field
through existing withInstanceIds / withEnvironment witheres. All existing
positional call-sites (SearchController, ExchangeMatchEvaluator,
ClickHouseSearchIndexIT, ClickHouseChunkPipelineIT) pass null for the new
slot.
Task 1.2 of docs/superpowers/plans/2026-04-22-per-exchange-exactly-once.md.
The evaluator-side wiring that actually supplies the cursor is Task 1.5.
b41f34c090