mod: Ext 10 — policy what-if / impact analysis, 260/260

mod/decision-diff compares one report's action under two rule sets;
mod/policy-impact batches a set and returns only the reports whose decision flips;
mod/impact-count / mod/impact-report summarize. Lets a mod team measure a policy
change's blast radius before shipping (e.g. removing spam-hide flips r1 hide→keep).
Pure SX over decide-report. +13 tests.

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

lib/mod/conformance.conf

@@ -18,6 +18,7 @@ PRELOADS=(
   lib/mod/offenders.sx
   lib/mod/quorum.sx
   lib/mod/trace.sx
+  lib/mod/whatif.sx
   lib/mod/lifecycle.sx
   lib/mod/audit.sx
   lib/mod/api.sx
@@ -38,4 +39,5 @@ SUITES=(
   "offenders:lib/mod/tests/offenders.sx:(mod-offenders-tests-run!)"
   "quorum:lib/mod/tests/quorum.sx:(mod-quorum-tests-run!)"
   "trace:lib/mod/tests/trace.sx:(mod-trace-tests-run!)"
+  "whatif:lib/mod/tests/whatif.sx:(mod-whatif-tests-run!)"
 )