Engine: snapshot multi-query reads and subscription groups at one revision (EN-15) #265

Goal. Coherent multi-view reads and subscriptions:

1. a snapshot read of several queries at one revision in one request (today a program with several ? queries replies with the last one only);
2. subscription groups: several queries in one subscribe frame, an atomic initial snapshot, one publication per commit that touches any member, all members exact at one revision, unchanged members marked.

Why. A caller that renders a state window from several views must see them all at the same revision. Today unchanged views publish nothing, so a subscriber cannot tell a view is still current at a revision.

Done when. Both are in the protocol and both SDKs, with tests for atomicity under concurrent writes. Part 1 can ship first.

Plan reference: EN-15. Part of the Engine production readiness milestone (the current focus; tracker #199). Rule for every engine PR: millisecond query and update latency must not regress, and the perf gate runs before the PR opens.