Session journal, context, and compaction contracts

A SessionJournal is the consumer-defined structural interface for committed session compare-and-append and snapshots. rho_memory_session_journal() is the process-local implementation. rho_agent() receives a journal explicitly; rho_sync_session() asynchronously projects an existing snapshot into an idle agent. Assistant streaming state remains in RhoAssistantTurn and only its terminal message is committed.

Details

The journal is append-only. Compare-and-append rejects a stale committed position before mutation. Message entries retain the complete interaction, reset entries begin a new active context without erasing history, and rho_build_agent_context() projects through the latest compaction entry and any explicit exclusions.

rho_compact() prepares a stable cut point, runs an asynchronous compactor, appends a semantic checkpoint, and invokes before/after policy methods. Automatic threshold compaction and one bounded provider-input recovery use the same protocol. Applications can supply another RhoCompactor class or specialize the public generics without replacing the agent loop.