define-metric

@veegee82

Use when the agent or the user wants to introduce a new measurable quantity as a first-class loss component — e.g., tracking cyclomatic complexity alongside test_pass_rate, or adding a domain-specific rubric to a reasoning chain. Forbids ad-hoc arithmetic on observations and forbids using a new metric as a load-bearing decision gate until calibration has passed. Extends LDD's loss framework to agent-defined objectives without touching LDD core.