ipc+mavweb: add revert/undo endpoint to void latest fact for a key

- New store.VoidLatestFact() method finds latest non-voided fact for
  a key and writes a void-marker row pointing at it (transactional).
- New IPC method MethodRevertFact with CoreAPI.RevertFact interface,
  storeAPI adapter, server dispatch, and client proxy.
- New HTTP endpoint POST /api/revert?key=<key> in mavweb.
- History page adds a 'revert' button per non-voided fact row with
  JS confirmation and optimistic UI (marks row voided on success).
- All existing store, IPC, and mavweb tests pass.

cmd/mavweb/history.html

@@ -4,7 +4,7 @@
 <meta name=viewport content="width=device-width,initial-scale=1">
 <title>maven · history</title>
 <style>
-  body{font:14px system-ui;margin:1.5rem;max-width:64rem}
+  body{font:14px system-ui;margin:1.5rem;max-width:70rem}
   h1{font-size:1.3rem;margin-bottom:.3rem}
   nav a{margin-right:1rem}
   table{border-collapse:collapse;width:100%;margin-top:.5rem}
@@ -16,12 +16,18 @@
   .val{font-family:monospace;word-break:break-all;max-width:20rem}
   .src{font-size:.85rem;color:#666}
   .count{color:#888;font-size:.85rem;margin-top:.3rem}
+  .revert-btn{font-size:.8rem;padding:.15rem .4rem;cursor:pointer}
+  .revert-btn:disabled{opacity:.4;cursor:not-allowed}
+  #msg{margin-top:.5rem;padding:.4rem;border-radius:.3rem;display:none}
+  .msg-ok{background:#e6ffed;display:block!important}
+  .msg-err{background:#ffe6e6;display:block!important}
 </style>
 <h1>maven · command history</h1>
 <nav><a href=/dash>dash</a> <a href=/tools>tools</a> <a href=/auth/passkey>passkey</a></nav>
 <p class=count>{{len .Facts}} facts shown (newest first)</p>
+<div id=msg></div>
 <table>
-<tr><th>when<th>kind<th>key<th>value<th>source<th>conf</tr>
+<tr><th>when<th>kind<th>key<th>value<th>source<th>conf<th></tr>
 {{range .Facts}}<tr{{if .VoidsID}} class=voided{{end}}>
 <td>{{if .VoidsID}}<span class=void-badge>⨯ voided</span>{{end}}{{.Ts.Format "2006-01-02 15:04"}}</td>
 <td>{{.Kind}}</td>
@@ -29,5 +35,23 @@
 <td class=val>{{.Value}}</td>
 <td class=src>{{.Source}}</td>
 <td>{{printf "%.2f" .Confidence}}</td>
+<td>{{if not .VoidsID}}<button class=revert-btn onclick="revert('{{.Key}}',this)">revert</button>{{end}}</td>
 </tr>{{end}}
 </table>
+<script>
+const msg=document.getElementById('msg');
+function revert(key,btn){
+  btn.disabled=true;
+  fetch('/api/revert',{method:'POST',headers:{'content-type':'application/x-www-form-urlencoded'},body:'key='+encodeURIComponent(key)})
+  .then(r=>r.json().then(d=>({ok:r.ok,data:d})))
+  .then(({ok,data})=>{
+    msg.className=ok?'msg-ok':'msg-err';
+    msg.textContent=ok?'reverted ✓':(data.error||'revert failed');
+    if(ok) btn.closest('tr').className='voided';
+  }).catch(e=>{
+    msg.className='msg-err';
+    msg.textContent='error: '+e;
+    btn.disabled=false;
+  });
+}
+</script>

cmd/mavweb/main.go

@@ -133,6 +133,9 @@ func main() {
 	mux.HandleFunc("/history", func(w http.ResponseWriter, r *http.Request) {
 		handleHistory(w, r, core)
 	})
+	mux.HandleFunc("/api/revert", func(w http.ResponseWriter, r *http.Request) {
+		handleRevert(w, r, core)
+	})
 	// ----- passkey (WebAuthn) endpoints -----
 	// Wired when both -core and a configured origin are present. The origin
 	// must match the browser's view of mavweb (e.g. https://maven.kvmx.ru).
@@ -385,6 +388,36 @@ func handleHistory(w http.ResponseWriter, r *http.Request, core ipc.CoreAPI) {
 	}
 }
 
+func handleRevert(w http.ResponseWriter, r *http.Request, core ipc.CoreAPI) {
+	if r.Method != http.MethodPost {
+		http.Error(w, "POST only", http.StatusMethodNotAllowed)
+		return
+	}
+	if core == nil {
+		http.Error(w, "revert disabled (no -core)", http.StatusServiceUnavailable)
+		return
+	}
+	key := strings.TrimSpace(r.FormValue("key"))
+	if key == "" {
+		http.Error(w, "key required", http.StatusBadRequest)
+		return
+	}
+	ctx := r.Context()
+	newID, err := core.RevertFact(ctx, key)
+	if err != nil {
+		log.Printf("revert %q: %v", key, err)
+		if errors.Is(err, ipc.ErrNoFact) {
+			http.Error(w, "no fact to revert", http.StatusNotFound)
+			return
+		}
+		http.Error(w, "revert failed", http.StatusBadGateway)
+		return
+	}
+	log.Printf("reverted fact for key=%s, new_id=%d", key, newID)
+	w.Header().Set("Content-Type", "application/json")
+	json.NewEncoder(w).Encode(map[string]any{"reverted": true, "new_id": newID})
+}
+
 // handleTools serves the enable surface (GET) and applies an enable (POST).
 // POST fields: name, cmd (space-separated argv), destructive (checkbox). cmd is
 // whitespace-split — argv with embedded spaces isn't supported (ponytail: no