Feature: Enhance Observable Callback Coverage

[LK-Simon]

Summary

Enhance ESPressio Command with observable coverage for externally meaningful command-registry lifecycle changes.

Scope

• Expose observation of command registration and unregistration, including scoped registration lifetime changes.
• Preserve existing execution callbacks, middleware, before/after hooks and command invocation semantics as the primary execution mechanism.
• Provide an optional ESPressio Event bridge for command-registry 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 as required.

Compatibility

Existing command registration and execution APIs must remain supported. Observable/Event integration must remain optional.