Add native coverage for ticking browser pages

Incremental-game style fixture whose AT-SPI leaves recreate every 250 ms, native navigation test, private-session AT-SPI daemon launch in the harness (no systemd, no host session required), and testing notes.

tests/browser-ticking.html

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+<!doctype html>
+<html lang="en"><meta charset="utf-8"><title>Cthulhu Browser Probe</title>
+<body>
+<label>Probe start <input autofocus></label>
+<h2>Rebuilt text region</h2>
+<p id="rebuilt"></p>
+<h2>Rebuilt container region</h2>
+<div id="container"></div>
+<h2>Renamed controls region</h2>
+<div id="renamed">
+  <button id="churn-one">Churn one cost 0</button>
+  <button id="churn-two">Churn two cost 0</button>
+</div>
+<h2>Final region</h2>
+<p>Text after ticking regions</p>
+<button id="final-action" onclick="document.title = 'Cthulhu Ticking Final Action'">Final action</button>
+<p id="final-text">Final paragraph</p>
+<script>
+"use strict";
+// Region A: every text leaf is recreated on each tick (same count and
+// order, new nodes), like incremental-game counters rebuilt per frame.
+let tickA = 0;
+const rebuilt = document.getElementById("rebuilt");
+function renderRebuilt() {
+  tickA++;
+  const lines = [];
+  for (let line = 1; line <= 3; line++) {
+    const span = document.createElement("span");
+    span.textContent = "Rebuilt line " + line + " value " + (tickA * 10 + line);
+    lines.push(span, document.createElement("br"));
+  }
+  rebuilt.replaceChildren(...lines);
+}
+renderRebuilt();
+setInterval(renderRebuilt, 300);
+
+// Region B: the whole container subtree, including a focusable button, is
+// recreated on each tick, destroying native focus anchors.
+let tickB = 0;
+const container = document.getElementById("container");
+function renderContainer() {
+  tickB++;
+  container.innerHTML = "";
+  const text = document.createElement("p");
+  text.textContent = "Container value " + tickB;
+  const button = document.createElement("button");
+  button.textContent = "Container action " + tickB;
+  button.addEventListener("click", () => {
+    document.title = "Cthulhu Ticking Container Action";
+  });
+  container.append(text, button);
+}
+renderContainer();
+setInterval(renderContainer, 400);
+
+// Region C: structure is stable; only button labels churn rapidly.
+let tickC = 0;
+setInterval(() => {
+  tickC++;
+  document.getElementById("churn-one").textContent = "Churn one cost " + tickC;
+  document.getElementById("churn-two").textContent = "Churn two cost " + tickC * 2;
+}, 50);
+</script>
+</body></html>