Elide low-value channel acknowledgements

[mostlydev]

Summary

• collapse exact short acknowledgements like ok/thanks/got it into sparse low_value_ack digest blocks by channel/hour
• preserve source message IDs on the sparse block for lookup/provenance
• keep context-bearing messages and hard events explicit
• document this follow-on Support compact/cursor-aware channel-awareness feeds for long channel tails #260 slice in the changelog

Stack

• Stacked on Collapse repeated channel decisions deterministically #326 / issue-260-repeated-decision-collapse

Tests

• go test ./examples/channel-memory
• go test ./...

Refs #260