mod: Phase 2 — evidence accumulation + proof trees + audit log, 60/60

Reports carry an :evidence list, asserted as evidence/3 facts; reviewer-remove
rule (highest precedence) lets human review override classification. Proof tree
built constructively by re-querying each rule body goal against the same DB with
the report id bound, so derivations carry real unification bindings. Append-only
audit log records decision + proof + evidence snapshot per decide, monotonic seq,
never mutates prior entries. +29 audit tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/mod/conformance.conf

@@ -13,9 +13,11 @@ PRELOADS=(
   lib/mod/schema.sx
   lib/mod/policy.sx
   lib/mod/engine.sx
+  lib/mod/audit.sx
   lib/mod/api.sx
 )
 
 SUITES=(
   "decide:lib/mod/tests/decide.sx:(mod-decide-tests-run!)"
+  "audit:lib/mod/tests/audit.sx:(mod-audit-tests-run!)"
 )