Decide-with-upstream scope: coprocessor / MPP / flashback / MVCC introspection / TiDB-domain hooks

[eduralph]

Plausibly TiDB-domain rather than general-client surface (incl. SplitRegion-and-scatter, lock-observer GC, binlog/schema/KV-filter hooks). File the scope question upstream; do not silently adopt or reject.

Roadmap §7