feat: implement granular section visibility control

Introduce the ability to reveal specific sections of a document
rather than entire files. This includes:

- Updating `revealedPaths` to map normalized file paths to sets of
  revealed section slugs.
- Adding `isPathRevealed` to handle visibility logic for connected
  clients and GMs.
- Enhancing `extractHeadings` to calculate `startLine` and `endLine`
  for each TOC node to support precise section identification.

src/components/journal/types/link.tsx

@@ -83,7 +83,16 @@ registerMessageType<LinkPayload>({
   reducer: (p) => {
     journalSetState(
       produce((s) => {
-        s.revealedPaths.add(p.path);
+        const key = p.path.replace(/^\.?\//, "").replace(/\.md$/, "");
+        if (!s.revealedPaths[key]) {
+          s.revealedPaths[key] = new Set();
+        }
+        if (p.section) {
+          s.revealedPaths[key].add(p.section);
+        } else {
+          // No section — reveal the whole article
+          s.revealedPaths[key].clear();
+        }
       }),
     );
   },