Single target detection

[LOCEANlloydizard]

This PR introduces the public API and output schema for single target detection! Ref #1532

It focuses on:

• defining the function signature and dispatcher integration
• validating required inputs and parameters
• returning a stable xr.Dataset with a target-based schema
• issuing warnings when optional metadata are missing

This provides a foundation for upcoming algorithm development and allows early review of the API design, data model, and test coverage!