mod/explain renders a decision's proof tree into legible text: action + rule,
evidence line, and each derivation goal with [proved]/[unproved] and the
unification bindings that satisfied it (e.g. {B=ann, N=3, S=dave}). Pure SX over
the Phase-2 proof data — the audit trail's 'why' made readable. +10 tests.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
