Feature: Enhance Observable Callback Coverage

[LK-Simon]

Summary

Enhance ESPressio ESP-Now with observable coverage for externally meaningful peer, transport and secure-session lifecycle changes.

Scope

• Expose peer lifecycle changes where managed by the library.
• Expose transport send success/failure and transport fault conditions where these represent lifecycle/operational state rather than replacing the primary receive path.
• Surface security-related lifecycle/failure observation consistently with ESPressio Security integration.
• Provide an optional ESPressio Event bridge for ESP-NOW lifecycle notifications without making ESPressio Event a mandatory dependency.
• Add representative tests/examples/documentation for the observable contract and Event integration.
• Update release metadata, dependency documentation and changelog, including updated upstream dependency versions.

Compatibility

Existing ESP-NOW transport and receive callback APIs must remain supported. Observable/Event integration must remain optional.