Repair Slack Scoped Historical Holes

Slack targeted channel repair can fetch a scoped source archive that contains messages older than the saved per-channel cursor. Those historical holes must be emitted during the repair; otherwise the run can succeed while retained records still miss source archive keys.

The Slack connector maintains per-channel message cursors so normal runs emit only messages newer than each channel's retained high-water mark. A channel-scoped repair run uses messages.resources to build a scoped slackdump archive for selected channels.

Debugging, replay, trace, projection, and operator-control surfaces that are useful for the reference implementation but are not part of core PDPP SHALL be explicitly marked as reference-only.