CompletionResolver.resolve() used to hand the caller a successful "" reply whenever a
turn's scrape came back empty (whether the read failed, or genuinely produced nothing),
making a lost turn indistinguishable from a real empty answer. It also had no way to
tell a crashed backend's near-instant BUSY -> DONE transition apart from a genuine
completion.
Add MIN_TURN_NANOS (2s), a named floor below which a completed turn is treated as a
crash signature and failed rather than resolved as a reply. Fail on any empty scrape
(read failure or a clean-but-empty read) instead of resolving with "". Both failures
name the member and carry whatever is on the pane for context.
Thread an injectable LongSupplier clock through CompletionResolver (matching the
SessionManager/MessageService nowNanos pattern) so the floor is testable without a
real sleep.
Dai Ha