feat: implement content reveal mechanism via DOM traversal

Add `addRevealedClasses` to apply `revealed` or `concealed` CSS classes
to article elements based on the current heading hierarchy and
revealed paths. Update `Article` to expose its DOM element via an
`onDom` callback.

src/components/stores/journalStream.ts

@@ -627,6 +627,54 @@ export function isPathRevealed(path: string, section?: string): boolean {
   return revealed.size > 0;
 }
 
+/**
+ * Walk every element in `root` and tag it as revealed or concealed.
+ *
+ * Headings (h1–h6) update a current-heading tracker. When a higher-level
+ * heading is encountered (e.g. h2 after h1), all sub-headings at deeper
+ * levels are cleared. Each non-heading element receives a `revealed` or
+ * `concealed` CSS class depending on whether any of its current heading
+ * ancestors is present in the `revealed` set.
+ */
+export function addRevealedClasses(
+  root: HTMLDivElement,
+  revealed: Set<string>,
+) {
+  if (!revealed) return;
+  // Current heading text for each level (h1..h6, index 1..6)
+  const cur: Record<number, string> = {};
+
+  const HEADING_TAGS = new Set(["H1", "H2", "H3", "H4", "H5", "H6"]);
+
+  const walk = (el: Element) => {
+    const tag = el.tagName.toUpperCase();
+
+    if (HEADING_TAGS.has(tag)) {
+      const level = Number(tag.charAt(1)); // 1-6
+      const text = el.textContent?.trim() ?? "";
+
+      // Update current heading at this level
+      cur[level] = text;
+      // Clear all deeper heading levels
+      for (let l = level + 1; l <= 6; l++) delete cur[l];
+    } else {
+      // Non-heading: check if any current heading ancestor is revealed
+      const isRevealed = Object.values(cur).some((h) => revealed.has(h));
+      el.classList.add(isRevealed ? "revealed" : "concealed");
+    }
+
+    // Recurse into children
+    for (const child of el.children) {
+      walk(child);
+    }
+  };
+
+  // Start walk from root's children (skip the container itself)
+  for (const child of root.children) {
+    walk(child);
+  }
+}
+
 /** Normalize a path for lookup: strip .md, leading ./ or / */
 function normalizePath(p: string): string {
   return p.replace(/^\.?\//, "").replace(/\.md$/, "");