Lucern

Commit a new belief (knowledge unit) to the reasoning graph. Like `git commit` — creates an atomic, traceable knowledge object with a prior. Creation requires a topic epistemic-node anchor and writes a `belongs_to` edge directly from belief node to topic node; orphan beliefs are invalid. Creation stores the vacuous opinion `(0, 0, 1, a)`; attach supporting or contradicting evidence with create_evidence or link_evidence_to_belief to record evidential updates.