Hand out the checkpoint when the source is what died - #141

This pull request adds no capability the code did not already have.

What I could not follow

3 calls resolve to a name whose type is written nowhere, so what they reach is not established here, and 2 more are in tests and benchmarks, which this report does not judge. Across everything it read that is 5 of 204, 2%.

• queue.send(IngestBatch(index++, offset, buffer)) — kdrant-core/src/commonMain/kotlin/dev/kdrant/Ingest.kt:173
• queue.send(IngestBatch(index, offset, buffer)) — kdrant-core/src/commonMain/kotlin/dev/kdrant/Ingest.kt:181
• pending.remove(nextExpected) — kdrant-core/src/commonMain/kotlin/dev/kdrant/Ingest.kt:279

If one of those reaches the network or starts a process, this report does not show it.

_{4 files read · 3131 ms · against 63c1168 · engine 1.4.1}