Deferred unique writes

[nick-potts]

1. New DeferBy Attribute:

#[DeferBy(property_names: string|array|null, name: string = 'EventClassName', replay_only: bool = false)]

2. New Verbs::defer() method:

Verbs::defer(State|string|iterable|null $unique_by, callable $callback, string $name = 'Default')

Key Benefits:

• Prevents duplicate operations by ensuring expensive operations only run once
• Operations are deferred until after everything is committed
• Can be scoped by state, property, or custom name

Examples of Usage:

1. Property-based uniqueness:

class UserPasswordUpdated extends Event 
{
    #[StateId(UserState::class)] 
    public int $user_id;

    #[DeferBy('user_id')]
    public function handle(UserState $user): void 
    {
        // Only the latest password is written
        $user->write();
    }
}

2. Named uniqueness:

class OrderCreated extends Event 
{
    #[DeferBy(null, name: 'daily_report')]
    public function handle(): void 
    {
        // Will only run once even if multiple orders are processed
        $this->generateDailyReports();
    }
}

3. Programmatic unique operations:

// Only runs once after commit
Verbs::defer($state, function() {
    $this->rebuildCache();
});

This is particularly useful for optimizing performance in scenarios where multiple events might trigger the same expensive operation, but running it once at the end is sufficient.

[nick-potts]

Potentially this should default to true. Event::commit() will default to returning values, and only replays will benefit initially.