merge: land axon-f0d728cd recursive transaction explain

5 ACs verified: explain_transaction_with_caller + explain_transaction_with_plan
public in axon-api/src/handler.rs; cargo test --workspace + clippy
green; GraphQL surface extends explainPolicy with transaction operations;
FEAT-031 spec doc updated.

Hybrid snapshot threading: subject (actor/bindings) constant across steps;
entity-data state evolves through HashMap shadow (create inserts, update/
patch insert post-call so explain sees pre-step current, delete removes)
giving the impact matrix full chain visibility without storage writes.

Cascade: unblocks axon-5c6e4a79 (UI transaction fixture editor) and
indirectly axon-84038791.

[axon-f0d728cd]

Author: easel