feat: Add type-safe state machines for read model construction

[jwilger]

Context

During the implementation of multi-stream query patterns (#148), expert agents suggested using type-safe state machines for constructing read models. This would make invalid state transitions impossible at compile time.

Current State

Read models allow invalid states (e.g., ended_at without a status, or active sessions with an ended_at timestamp).

Proposed Solution

Implement type-safe state machines using phantom types:

pub struct SessionSummary<S> {
    session_id: SessionId,
    user_id: UserId,
    // ... other fields
    _state: PhantomData<S>,
}

pub struct Active;
pub struct Ended;

impl SessionSummary<Active> {
    pub fn end(self, ended_at: Timestamp, status: SessionStatus) -> SessionSummary<Ended> {
        // State transition
    }
}

impl SessionSummary<Ended> {
    // Methods only available for ended sessions
    pub fn duration(&self) -> Duration {
        // ...
    }
}

Benefits

• Compile-time guarantees about state validity
• Self-documenting API
• Impossible to misuse
• Clear state transitions

Tasks

• Design state machines for each read model
• Implement phantom type pattern
• Update projection builders to use state machines
• Add tests for state transitions
• Document the pattern for future developers

Related

• Depends on refactor: Replace mutable read model methods with pure functions #160 (pure functions refactoring)

Labels

• enhancement
• type-safety