// Package chatstate owns the durable execution-state transitions for
// the chatd subsystem. It implements the chat execution state model.
//
// The package exposes two top-level entry points:
//
//   - [CreateChat] creates a brand new chat with its initial history
//     in a single transaction. It is standalone because no chat-scoped
//     state machine instance can exist before the chat row is written.
//   - [ChatMachine] wraps an existing chat. Callers use it to apply
//     one or more transitions atomically via [ChatMachine.Update], or
//     to read related rows while holding the chat row lock via
//     [ChatMachine.Lock].
//
// Every successful [ChatMachine.Update] call locks the chat row,
// advances `snapshot_version` exactly once, applies transition methods
// in order, and (on commit) publishes a single typed `chat:update`
// pubsub message describing the post-transition snapshot. Optional
// `chat:ownership` hints are published only when the post-transition
// state is runnable and ownership is missing or stale. Stream side
// effects are handled by `chat:update` consumers, and ownership hints
// wake chat workers.
//
// Transition methods are explicit, typed wrappers around the durable
// mutations needed to move between states. Each transition reads the
// current chat row and queue cardinality, classifies the resulting
// execution state, validates it against the transition model, and
// rejects with an [*TransitionError] wrapping [ErrTransitionNotAllowed]
// when the transition is not legal from that state. The package owns
// transition validation, durable chat row and queue mutations, and
// post-commit pubsub publication.
package chatstate