mod: Ext 4 — report linking + dedup (Prolog-backed retrieval), 176/176

mod/related-ids and mod/reporters-of find reports about a subject via a Prolog
relational query (report(Id, _, 'subject')) — the policy substrate reused for
retrieval. mod/dedup-reports collapses identical reports by a normalized
reporter|subject|reason key; mod/distinct-reporters-of counts unique reporters.
Own suite (tests/link.sx). +12 tests.

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

plans/mod-on-sx.md

@@ -16,7 +16,7 @@ federation extension.
 
 ## Status (rolling)
 
-`bash lib/mod/conformance.sh` → **164/164** (roadmap done + extensions in progress)
+`bash lib/mod/conformance.sh` → **176/176** (roadmap + 4 extensions complete)
 
 ## Ground rules
 
@@ -147,10 +147,22 @@ lib/mod/fed.sx
   derivation goal-by-goal with `[proved]`/`[unproved]` marks and unification
   bindings. E.g. `Report rc: escalate (rule: repeated-escalate)` … `[proved]
   report(rc, B, S), report_count(S, N), N >= 3  {B=ann, N=3, S=dave}`.
-- [ ] Ext 4 — report linking / dedup (relations between reports about one subject)
+- [x] **Ext 4 — report linking / dedup** (`lib/mod/link.sx`, +12). `mod/related-ids`
+  and `mod/reporters-of` find reports about a subject via a Prolog relational query
+  (`report(Id, _, 'subject')`) — the policy substrate reused for retrieval.
+  `mod/dedup-reports` collapses identical reports (reporter|subject|reason key,
+  case-insensitive); `mod/distinct-reporters-of` counts unique reporters.
 
 ## Progress log
 
+- **Ext 4 — report linking / dedup, 176/176** (+12). Relational retrieval
+  (`related-ids`, `reporters-of`) reuses the Prolog substrate for *querying* report
+  clusters, not just deciding them — `report(Id, _, 'subject')` by unification.
+  Dedup is pure SX over a normalized link key. Own suite (`tests/link.sx`) — going
+  forward, new extensions get their own test file rather than growing
+  `extensions.sx`. With roadmap + 4 extensions the subsystem now spans schema →
+  policy DSL (6 condition types) → engine + proofs → audit → lifecycle →
+  federation → explanation → linking, all on the green `lib/prolog` substrate.
 - **Ext 3 — proof explanation, 164/164** (+10). `mod/explain` turns the Phase-2
   proof tree into human-readable text — the audit trail's "why" made legible. Pure
   SX over existing decision data; no engine change. Renders unification bindings