mod: Ext 3 — human-readable proof explanation (mod/explain), 164/164

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>

lib/mod/conformance.conf

@@ -13,6 +13,7 @@ PRELOADS=(
   lib/mod/schema.sx
   lib/mod/policy.sx
   lib/mod/engine.sx
+  lib/mod/explain.sx
   lib/mod/lifecycle.sx
   lib/mod/audit.sx
   lib/mod/api.sx