mod: Ext 9 — policy dry-run trace diagnostics, 247/247

mod/trace-rules evaluates a report against every rule, returning each rule's
proved/unproved status + goal-by-goal derivation (an unproved rule shows which
goal failed). mod/first-proved = winner (matches engine precedence, cross-checked),
mod/proved-rules the firing set, mod/trace-report a [fires]/[ - ] rendering.
Answers 'why didn't my rule fire?' without instrumenting the engine. +15 tests.

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

lib/mod/trace.sx

@@ -0,0 +1,56 @@
+;; lib/mod/trace.sx — policy dry-run diagnostics.
+;;
+;; decide-report returns the winning rule; a policy author debugging "why didn't
+;; my rule fire?" needs the whole picture. mod/trace-rules evaluates a report
+;; against every rule and reports each rule's proved/unproved status plus its
+;; goal-by-goal derivation — so an unproved rule shows exactly which goal failed.
+;; The winner is the first proved rule (same precedence as the engine).
+
+(define
+  mod/trace-rules
+  (fn
+    (r reports rules)
+    (let
+      ((count (mod/report-count (mod/report-about r) reports))
+        (id (mod/report-id r)))
+      (let
+        ((db (pl-load (mod/build-program r count rules))))
+        (let
+          ((proved-names (map (fn (s) (dict-get s "Rule")) (pl-query-all db (str "policy_action(" id ", _, Rule)")))))
+          (map
+            (fn (rule) (let ((nm (mod/rule-name rule))) {:proved (mod/member? nm proved-names) :goals (mod/proof-goals db id (mod/rule-when rule)) :action (mod/rule-action rule) :rule nm}))
+            rules))))))
+
+(define
+  mod/first-proved
+  (fn
+    (trace)
+    (reduce
+      (fn (acc t) (if (nil? acc) (if (get t :proved) t acc) acc))
+      nil
+      trace)))
+
+(define
+  mod/proved-rules
+  (fn
+    (trace)
+    (reduce
+      (fn
+        (acc t)
+        (if (get t :proved) (append acc (list (get t :rule))) acc))
+      (list)
+      trace)))
+
+(define
+  mod/trace-row
+  (fn
+    (t)
+    (str
+      (if (get t :proved) "[fires] " "[  -  ] ")
+      (get t :rule)
+      " → "
+      (get t :action))))
+
+(define
+  mod/trace-report
+  (fn (trace) (mod/join-with "\n" (map mod/trace-row trace))))