Upcaster behaviour for event schema_version evolution (D3-69).

Phase 3 ships every event at schema_version: 1 with identity upcasters. This module establishes the contract for future breaking changes — when a payload shape changes, the new schema module will ship with a real upcast/1 that maps the old payload into the new shape so historical rows remain queryable on the read path.

Contract

• upcast/1 takes a raw data map as stored in the accrue_events.data jsonb column.
• Returns {:ok, upcasted_map} on success or {:error, term()} when the payload is malformed beyond recovery.

Why a behaviour, not a protocol

Events are stored by string type (e.g. "subscription.created") and schema_version integer — there is no struct in flight when the read path needs to upcast, so a protocol wouldn't dispatch. The registry lookup in Accrue.Events.Schemas.for/1 returns the module, and the read path calls mod.upcast(data) directly.