signal_set API needs tidying

[vinniefalco]

A lot of dual-error functions (error + exception). The error should probably be returned not taken as an outparam.

[sgerbino]

This issue consolidates API design concerns, thread safety questions, and documentation needs for the signal/slot implementation.

Discussion Points

Signal/Slot API Design

• Remove unnecessary signal registration overloads in favor of a minimalist approach—add features only when users specifically request them
• Address current API issues including unnecessary namespace repetition and problematic error handling approaches
• Standardize on system::result return type and eliminate dual API implementations

Thread Safety and Implementation Concerns

• Clarify thread safety guarantees for signal operations (e.g., can methods like cancel be called from any thread?)
• Specify precise thread safety guarantees for all shared object operations