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/tests/extensions.sx

@@ -5,6 +5,7 @@
 ;; Ext 2: weighted/aggregate evidence scoring (:score-at-least) + report signals.
 ;;        Many low-confidence signals accumulate past a threshold via Prolog
 ;;        aggregate_all(sum(W), ...).
+;; Ext 3: human-readable proof explanation (mod/explain) over the proof tree.
 ;; Demonstrated with custom rule sets so the default policy (and its conformance
 ;; tests) stays untouched.
 
@@ -247,4 +248,66 @@
   (mod/cond->goal (list :score-at-least 5) "Id")
   "aggregate_all(sum(W), signal(Id, _, W), T), T >= 5")
 
+;; ── Ext 3: human-readable proof explanation ──
+
+(define mod-ext-spam-explain (mod/explain mod-ext-dec))
+
+(mod-ext-test!
+  "explain mentions the report id"
+  (mod/str-contains? mod-ext-spam-explain "Report p1")
+  true)
+(mod-ext-test!
+  "explain mentions the action"
+  (mod/str-contains? mod-ext-spam-explain "hide")
+  true)
+(mod-ext-test!
+  "explain mentions the rule"
+  (mod/str-contains? mod-ext-spam-explain "spam-unverified-hide")
+  true)
+(mod-ext-test!
+  "explain marks proved goals"
+  (mod/str-contains? mod-ext-spam-explain "[proved]")
+  true)
+(mod-ext-test!
+  "explain renders the evidence line"
+  (mod/str-contains? mod-ext-spam-explain "Evidence: spam")
+  true)
+
+;; count-rule explanation shows the unification bindings
+(define mod-ext-rep-r (mod/mk-report "rc" "ann" "dave" "off-topic"))
+(define
+  mod-ext-rep-d
+  (mod/decide-report
+    mod-ext-rep-r
+    (list mod-ext-rep-r mod-ext-rep-r mod-ext-rep-r)
+    mod/default-rules))
+(define mod-ext-rep-explain (mod/explain mod-ext-rep-d))
+(mod-ext-test!
+  "explain shows binding N=3"
+  (mod/str-contains? mod-ext-rep-explain "N=3")
+  true)
+(mod-ext-test!
+  "explain shows subject binding"
+  (mod/str-contains? mod-ext-rep-explain "dave")
+  true)
+
+;; explain-goal direct: unproved goal gets [unproved]
+(mod-ext-test!
+  "explain-goal marks unproved"
+  (mod/str-contains? (mod/explain-goal {:solved false :goal "attr(x, foo)" :bindings {}}) "[unproved]")
+  true)
+;; explain-binds renders key=value pairs
+(mod-ext-test!
+  "explain-binds renders pair"
+  (mod/explain-binds {:N "3"})
+  "N=3")
+;; no-evidence decision says (none)
+(define
+  mod-ext-keep-d
+  (mod/decide-report mod-ext-clean (list mod-ext-clean) mod-ext-rules))
+(mod-ext-test!
+  "explain (none) for empty evidence"
+  (mod/str-contains? (mod/explain mod-ext-keep-d) "Evidence: (none)")
+  true)
+
 (define mod-extensions-tests-run! (fn () {:failures mod-ext-failures :total mod-ext-count :passed mod-ext-pass :failed mod-ext-fail}))