feat(control): SUIVANT queued-load button

Add a SUIVANT button to the matrix persist bar that arms the selected
preset to load at the next loop wrap (/matrix/loadnext), with an armed
pulse + queued-name label driven by the /matrix/queued feedback.

No jsdom unit test: the project's web tests cover pure logic modules, not
main.js DOM wiring (jsdom is not a dependency); the button is verified by
the live smoke. node --check + the existing 29-test suite pass.

web_realart/public/control/control.css

@@ -127,6 +127,9 @@ button.queued { animation: queued-blink 600ms ease-in-out infinite alternate; }
   border: 1px solid #335; border-radius: 6px; padding: 7px 6px; font-size: 14px;
   cursor: pointer; -webkit-appearance: none; appearance: none; }
 .mat-action-btn { width: auto; padding: 8px 14px; font-size: 14px; flex-shrink: 0; }
+.mat-action-btn.armed { background: #2a4d2a; color: #9f9; border-color: #4a4;
+  animation: matArmedPulse 1s ease-in-out infinite; }
+@keyframes matArmedPulse { 0%,100% { opacity: 1; } 50% { opacity: .55; } }
 .mat-refresh-btn { width: auto; padding: 8px 12px; font-size: 16px; flex-shrink: 0;
   background: #1a2a1a; border-color: #2a4a2a; color: #4d9; }
 .mvoice-label { position: sticky; left: 0; z-index: 2; background: #111; }

web_realart/public/control/index.html

@@ -169,6 +169,7 @@
     <button id="matrix-save" class="mat-action-btn">SAUVER</button>
     <select id="matrix-list" class="matrix-list-select"></select>
     <button id="matrix-load" class="mat-action-btn">CHARGER</button>
+    <button id="matrix-loadnext" class="mat-action-btn" title="Charge a la fin de la boucle">SUIVANT</button>
     <button id="matrix-refresh" class="mat-refresh-btn" title="Rafraichir la liste">&#8635;</button>
   </div>
   <button id="matrix-loop-full" class="mat-action-btn mat-loop-full-btn">BOUCLE COMPLETE</button>

web_realart/public/control/js/main.js

@@ -116,6 +116,19 @@ document.addEventListener("DOMContentLoaded", () => {
     const n = document.getElementById("matrix-list").value;
     if (n) send("/matrix/load", n);
   });
+  // Queued load: arm the selected preset to swap at the next loop wrap.
+  const matLoadNext = document.getElementById("matrix-loadnext");
+  if (matLoadNext) matLoadNext.addEventListener("click", () => {
+    const n = document.getElementById("matrix-list").value;
+    if (n) send("/matrix/loadnext", n);
+  });
+  on("/matrix/queued", (args) => {
+    const name = (args && args[0] != null) ? String(args[0]) : "";
+    if (matLoadNext) {
+      matLoadNext.classList.toggle("armed", name.length > 0);
+      matLoadNext.textContent = name.length > 0 ? "SUIVANT → " + name : "SUIVANT";
+    }
+  });
   const matRefresh = document.getElementById("matrix-refresh");
   if (matRefresh) matRefresh.addEventListener("click", () => send("/matrix/list"));
 });