mod: Ext 18 — ergonomic defrule / ruleset surface, 375/375

mod/defrule collects trailing conditions via &rest; mod/ruleset assembles rules.
No macro needed — conditions are plain data, fn supports &rest here. Produces
structurally identical rules to mk-rule (asserted) and works in the engine
unchanged. Closes the roadmap's original defrule surface. +11 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` → **364/364** (roadmap + 17 extensions complete)
+`bash lib/mod/conformance.sh` → **375/375** (roadmap + 18 extensions complete)
 
 ## Ground rules
 
@@ -147,6 +147,11 @@ 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}`.
+- [x] **Ext 18 — ergonomic defrule / ruleset** (`lib/mod/defrule.sx`, +11). The
+  roadmap's `(defrule …)` surface, done with `&rest` variadics (no macro needed —
+  conditions are already plain data): `mod/defrule` collects trailing conditions,
+  `mod/ruleset` assembles rules. Produces structurally identical rules to `mk-rule`
+  and works in the engine unchanged.
 - [x] **Ext 17 — per-domain policy registry** (`lib/mod/policies.sx`, +14).
   `mod/register-policy! domain rules` + `mod/decide-in domain r reports` give each
   rose-ash domain (blog/market/events/…) its own rule set; unregistered domains
@@ -229,6 +234,11 @@ lib/mod/fed.sx
 
 ## Progress log
 
+- **Ext 18 — ergonomic defrule / ruleset, 375/375** (+11). Closes the roadmap's
+  original `defrule` surface. `fn` supports `&rest` here, and conditions evaluate
+  to plain data, so no macro is needed — variadic functions give the ergonomics
+  safely. Equivalence to `mk-rule` is asserted, so it's pure sugar with no new
+  semantics.
 - **Ext 17 — per-domain policy registry, 364/364** (+14). Multi-tenant policy:
   the engine already took `rules` as a parameter, so domain-scoping is just a
   registry + a default fallback — no engine change. Makes the whole policy