fix: pierce new-reddit (shreddit) shadow DOM when collecting comment text (closes task/handle-shreddit-shadow-dom)

Source: task/handle-shreddit-shadow-dom (plan/steward-linker-design) — the TreeWalker only walked light DOM, so new-reddit comment bodies inside shadow roots were missed.

- collectTextNodes now walks the comment-tree root's light DOM AND recursively descends into the open shadow roots of elements within that subtree, so comment text is reached whether reddit slots it (light DOM) or encapsulates it (shadow DOM).
- Scoped to getRoot()'s comment tree (never the whole document's shadow roots); bounded by a depth cap; a no-op on old reddit (no shadow roots).
- Slotted light-DOM nodes are not double-collected (a shadow walker sees the shadow tree's own nodes, not a <slot>'s assigned light nodes); a Set de-dupes by node identity belt-and-braces.
- All existing guards preserved (inSkippable / rsl-link skip). README + AGENTS document the support.
- Limitation: CLOSED shadow roots are unreachable from a content script and stay unlinked.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

extension/content.js

@@ -30,10 +30,27 @@
     return false;
   }
 
-  function collectTextNodes(root) {
-    const nodes = [];
-    const walker = document.createTreeWalker(root, NodeFilter.SHOW_TEXT, {
+  // New reddit's shreddit-* web components may render comment bodies inside an OPEN shadow
+  // root. A TreeWalker can't cross a shadow boundary, so collectTextNodes walks `root`'s light
+  // DOM AND, recursively, every open shadow root of elements within that subtree — so it reaches
+  // the comment text whether reddit slots it (light DOM) or encapsulates it (shadow DOM). Old
+  // reddit has no shadow roots, so the recursion is a no-op there. Scoped to the comment-tree
+  // `root` (getRoot()) — it never walks the whole document's shadow roots, which would pull in
+  // reddit's chrome/UI text and be unbounded. A slotted light-DOM node is reached only by the
+  // light walk: a TreeWalker over a shadow root walks the shadow tree's own nodes, not a <slot>'s
+  // assigned (light) nodes, so it isn't double-collected — a Set keyed on node identity is a
+  // belt-and-braces guard. Closed shadow roots (`.shadowRoot === null`) stay unreachable.
+  const MAX_SHADOW_DEPTH = 50; // bound the shadow-host recursion; a real comment tree is far shallower.
+
+  function collectInto(root, nodes, seen, depth) {
+    if (depth > MAX_SHADOW_DEPTH) return;
+    // SHOW_TEXT to gather candidate text; SHOW_ELEMENT so we can spot shadow hosts to recurse into.
+    const walker = document.createTreeWalker(root, NodeFilter.SHOW_TEXT | NodeFilter.SHOW_ELEMENT, {
       acceptNode(node) {
+        if (node.nodeType === Node.ELEMENT_NODE) {
+          // Elements are visited only to discover shadow hosts; never collected as text.
+          return node.shadowRoot ? NodeFilter.FILTER_ACCEPT : NodeFilter.FILTER_SKIP;
+        }
         const v = node.nodeValue;
         if (!v || v.length > 5000 || !/[A-Za-z]/.test(v)) return NodeFilter.FILTER_REJECT;
         if (inSkippable(node)) return NodeFilter.FILTER_REJECT;
@@ -41,7 +58,20 @@
       },
     });
     let n;
-    while ((n = walker.nextNode())) nodes.push(n);
+    while ((n = walker.nextNode())) {
+      if (n.nodeType === Node.ELEMENT_NODE) {
+        // Open shadow host: descend into its shadow tree (and its descendants' shadow trees).
+        collectInto(n.shadowRoot, nodes, seen, depth + 1);
+      } else if (!seen.has(n)) {
+        seen.add(n);
+        nodes.push(n);
+      }
+    }
+  }
+
+  function collectTextNodes(root) {
+    const nodes = [];
+    collectInto(root, nodes, new Set(), 0);
     return nodes;
   }