Simulate presence channel membership lifecycle

[luisdalmolin]

Problem

The package can identify presence-channel registrations but cannot drive Echo's here(), joining(), or leaving() callbacks or maintain pusher-js presence member state.

Proposed direction

Simulate the real Pusher presence protocol through channel handleEvent() calls using subscription-succeeded, member-added, and member-removed payloads.

Acceptance criteria

• Tests can seed the initial presence member list.
• Tests can simulate one member joining and leaving.
• Echo's normal here(), joining(), and leaving() callbacks receive the same shapes they receive from pusher-js.
• The underlying pusher-js member registry stays consistent after every transition.
• Real Laravel Echo/pusher-js browser coverage verifies the protocol shapes.