Snapshot FTMS callback state before diffing

[michaelw]

Summary

• snapshot public properties/settings mappings before returning read-only views
• snapshot realtime update state before zero filtering and diffing
• use a local sentinel diff so callback payloads are built only from stable current data

Why

This is a defensive concurrency hardening change. The previous mapping proxies and updater diff code could expose or iterate live mutable dictionaries while callbacks or notification handling were still updating them. Returning snapshots and diffing copied state prevents retained views and callback payloads from changing underneath callers.

Tests

• hassio-dev exec uv run --project repos/python-pyftms --python 3.13 pytest repos/python-pyftms/tests/test_manager.py repos/python-pyftms/tests/test_updater.py
• hassio-dev exec uv run --project repos/python-pyftms --python 3.13 pytest repos/python-pyftms/tests