mod: Ext 6 — strictest-wins decision strategy + action severity, 204/204

mod/decide-strictest collects every proven rule (pl-query-all) and applies the
harshest action by mod/action-severity (keep<escalate<hide<remove<ban), an
alternative to the engine's first-match precedence. Diverges from first-match
exactly when rule order and severity disagree. Same decision shape + :strategy;
engine untouched. Own suite. +14 tests.

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

lib/mod/conformance.conf

@@ -14,6 +14,7 @@ PRELOADS=(
   lib/mod/policy.sx
   lib/mod/engine.sx
   lib/mod/explain.sx
+  lib/mod/severity.sx
   lib/mod/lifecycle.sx
   lib/mod/audit.sx
   lib/mod/api.sx
@@ -30,4 +31,5 @@ SUITES=(
   "extensions:lib/mod/tests/extensions.sx:(mod-extensions-tests-run!)"
   "link:lib/mod/tests/link.sx:(mod-link-tests-run!)"
   "lint:lib/mod/tests/lint.sx:(mod-lint-tests-run!)"
+  "severity:lib/mod/tests/severity.sx:(mod-severity-tests-run!)"
 )