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();
+        }
       }),
     );
   },

src/components/stores/journalStream.ts

@@ -29,10 +29,12 @@ export interface JournalStreamState {
   /** Last sequence number per sender */
   senderSeq: Record<string, number>;
   /**
-   * Paths revealed by link messages.
+   * Paths and sections revealed by link messages.
+   * Key: normalized path (no .md). Value: set of revealed section slugs.
+   * An empty set means the whole article is revealed.
    * Populated during hydration and live receipt via the type's reducer.
    */
-  revealedPaths: Set<string>;
+  revealedPaths: Record<string, Set<string>>;
   /** MQTT connection status */
   connected: boolean;
   /** Granular connection state for UI indicators */
@@ -101,7 +103,7 @@ const [state, setState] = createStore<JournalStreamState>({
   sessionName: null,
   messages: [],
   senderSeq: {},
-  revealedPaths: new Set(),
+  revealedPaths: {},
   connected: false,
   connectionStatus: "disconnected",
   connectionError: null,
@@ -604,6 +606,32 @@ export function visibleMessages(): StreamMessage[] {
   return state.messages.filter((m) => !m.reverted);
 }
 
+/**
+ * Check whether a path (and optionally section) is revealed.
+ * GM and disconnected clients see everything.
+ * Non-GM connected clients only see explicitly revealed content.
+ */
+export function isPathRevealed(path: string, section?: string): boolean {
+  if (!state.connected) return true;
+  if (state.myRole === "gm") return true;
+
+  const normalized = normalizePath(path);
+  const revealed = state.revealedPaths[normalized];
+  if (!revealed) return false;
+
+  // Whole article revealed (empty set)
+  if (revealed.size === 0) return true;
+  // Section-specific check
+  if (section) return revealed.has(section);
+  // No section asked — at least some sections are revealed, article is partially visible
+  return revealed.size > 0;
+}
+
+/** Normalize a path for lookup: strip .md, leading ./ or / */
+function normalizePath(p: string): string {
+  return p.replace(/^\.?\//, "").replace(/\.md$/, "");
+}
+
 export function useJournalStream() {
   return state;
 }