The steady-state turn only bumps updated_at/last_prompt_tokens on one
routing entry, but persisted it through the full index rewrite twice
per turn (get_or_create_session's healthy-path bump + update_session):
every entry re-serialized, DELETE+INSERT of every gateway_routing row,
and a multi-MB sessions.json dump+fsync — ~50ms p50 at ~1,100 routing
keys in production, out of ~175ms total per-turn gateway persistence.
Metadata-only saves now UPSERT the single row via the existing
HermesDB.save_gateway_routing_entry (<1ms). Structural transitions
(create/recover/reset/switch/prune, compression-tip heals) keep the
full rewrite, which also refreshes the legacy sessions.json mirror.
Correctness: each fast save allocates a per-entry revision from the
routing generation counter under _lock, so fast and full snapshots are
totally ordered by number. Under _save_lock the UPSERT is skipped when
a newer full snapshot or a newer fast save of the same key has already
persisted, and a delayed full rewrite folds in fast records serialized
after its snapshot before writing — an older snapshot can never
overwrite a newer one, in either direction. update_session snapshots
peer fields under _lock so a concurrent reset cannot record a torn
peer row; no DB or a failed UPSERT falls back to the full rewrite so
DB-less installs keep sessions.json durable every turn.
Soju06