Compare terminal Jepsen cursors with origin stream heads - #21

Problem

The terminal invariant accepted any nonnegative receiver cursor under current authority. A receiver at cursor 101 for an origin whose head was 1 could therefore report healthy, and a missing required cursor was invisible to that check.

Fix

Capture each origin's active epochs and fully applied per-shard stream heads independently, alongside every receiver cursor and its actual lane. The independent checker derives admitted streams from both sides' active clusters and requires exact terminal positions. Head-zero streams permit an absent cursor or a zero admission marker; stale, retired, wrong-lane and otherwise extra cursors are rejected, including extra zero cursors.

Stream evidence participates in terminal stability comparisons. Existing internal invariants remain in place, and snapshot clients collect evidence without adding cross-node calls to Group shards.

Supporting information

Terminal captures now require stream evidence; older histories cannot qualify without it. Generation and epoch references use opaque external-term encodings to retain identity across EDN transport.

Executable capture qualification uses three real Group nodes and the actual snapshot-to-EDN boundary, including the cursor-101/head-1 counterexample, missing required cursors, no-write streams, cluster closure, instance restart and retirement. It is separate from the Java-only pure checker and included in the normal Mix gate.