Session chain hash after this movement.
The movement's idempotency key, unique across all sessions.
Session chain hash before this movement; the genesis value (64 zeros) for the first.
Epoch ms the movement was accepted.
Position in the session chain, from 0.
One accepted session movement: a balanced set of legs not yet posted to the ledger, made ledger-final at settle. Sessions are economy-tier objects keyed by an opaque scope (a game-world instance is one natural key; see src/netting.ts for the tier boundary).
prevHash/hashchain the session's movements and the settlement posting anchors the final head, so tamper-evidence extends to every movement.