mod: Ext 10 — policy what-if / impact analysis, 260/260

mod/decision-diff compares one report's action under two rule sets;
mod/policy-impact batches a set and returns only the reports whose decision flips;
mod/impact-count / mod/impact-report summarize. Lets a mod team measure a policy
change's blast radius before shipping (e.g. removing spam-hide flips r1 hide→keep).
Pure SX over decide-report. +13 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` → **247/247** (roadmap + 9 extensions complete)
+`bash lib/mod/conformance.sh` → **260/260** (roadmap + 10 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 10 — policy what-if / impact** (`lib/mod/whatif.sx`, +13).
+  `mod/decision-diff` compares one report's action under two rule sets;
+  `mod/policy-impact` runs a batch and returns only the reports whose decision
+  flips; `mod/impact-count` / `mod/impact-report` summarize. Lets a team measure a
+  policy change before shipping it (e.g. "removing spam-hide flips r1 hide→keep").
 - [x] **Ext 9 — policy dry-run trace** (`lib/mod/trace.sx`, +15). `mod/trace-rules`
   evaluates a report against every rule and returns each rule's proved/unproved
   status + its goal-by-goal derivation, so an unproved rule shows which goal
@@ -186,6 +191,12 @@ lib/mod/fed.sx
 
 ## Progress log
 
+- **Ext 10 — policy what-if / impact, 260/260** (+13). Decisions are now
+  comparable across rule sets — diff one report, or batch a whole set and surface
+  only the flips. Pure SX over `decide-report`, no engine change. Closes the
+  policy-authoring loop alongside lint (Ext 5) and trace (Ext 9): lint checks
+  well-formedness, trace explains one report, what-if measures a change's blast
+  radius before it ships.
 - **Ext 9 — policy dry-run trace, 247/247** (+15). Whole-rule-set diagnostics over
   the proof machinery: every rule's fire/no-fire and the goal that decided it. The
   winner agrees with `decide-report` by construction (first proved = pl-query-one),