Complete S7CommPlus data subscriptions with notification receive and lifecycle handling

[gijzelaerr]

Problem

The sync and async clients expose create_subscription(), and its documentation directs users to receive_notification(), but no such method exists. There is no notification dispatcher, decoding path, credit replenishment or reconnect lifecycle.

The public API therefore advertises a feature that cannot be consumed.

Alarm subscriptions remain tracked separately in #794.

Scope

• Build on the response/notification dispatcher rather than reading directly from the request path.
• Decode data-change notifications and associate them with the correct subscription object/reference IDs.
• Expose a bounded sync iterator/callback API and async iterator/queue API.
• Handle finite credit replenishment.
• Buffer a notification that arrives before the CreateObject response registers its subscription ID.
• Delete subscriptions cleanly and prevent stale notifications from being delivered after ID reuse.
• Define behavior on disconnect and optional resubscription after reconnect.

Acceptance criteria

• A created subscription can deliver decoded values through a documented public API.
• Notifications cannot satisfy ordinary requests.
• Initial, burst and out-of-order lifecycle cases are covered by deterministic tests.
• Queues are bounded and expose overflow/missed-update diagnostics.
• Delete/disconnect releases PLC objects and local receiver resources.
• Documentation no longer references a nonexistent method.

Dependencies

• Response correlation and unsolicited-frame routing
• Typed symbolic tag descriptors for name-based subscriptions