feat(obsidian): add FMG ID index for instant note lookups

Problem: Every time you click on a burg/marker, it had to scan through
all 13,496 vault files looking for a matching fmg-id. This was extremely
slow and made the feature unusable for large vaults.

Solution: Implement a smart index that maps fmg-id to file paths.

How it works:
- Index stored in memory (fmgIdIndex object)
- Persisted to localStorage (survives page reloads)
- Loaded on init, saved on changes
- When looking up by fmg-id:
  1. Check index first (instant!)
  2. If found, verify file still exists and ID matches
  3. If not in index, search vault and add to index
- Automatically updates when notes are created/saved through FMG
- Handles stale entries (file deleted/modified)

Performance improvement:
- Before: O(n) - scan all 13k files (very slow)
- After: O(1) - instant lookup from index
- First click on burg: May need to search (builds index)
- Second click on same burg: Instant! Opens note directly

This makes the Obsidian integration actually usable. Create a note once
for a burg, and every time you click that burg again, it opens instantly.

modules/ui/obsidian-notes-editor.js

@@ -216,6 +216,13 @@ async function promptCreateNewNote(elementId, elementType, coordinates) {
 
             const {frontmatter} = ObsidianBridge.parseFrontmatter(template);
 
+            // Add to FMG ID index for instant future lookups
+            const fmgId = frontmatter["fmg-id"] || frontmatter.fmgId;
+            if (fmgId) {
+              ObsidianBridge.addToFmgIdIndex(fmgId, notePath);
+              INFO && console.log(`New note added to index: ${fmgId} → ${notePath}`);
+            }
+
             resolve({
               path: notePath,
               name,
@@ -590,6 +597,17 @@ async function saveObsidianNote() {
 
   try {
     await ObsidianBridge.updateNote(path, content);
+
+    // Update the FMG ID index if this note has an fmg-id
+    if (elementId) {
+      const {frontmatter} = ObsidianBridge.parseFrontmatter(content);
+      const fmgId = frontmatter["fmg-id"] || frontmatter.fmgId;
+      if (fmgId) {
+        // Add to index using internal method
+        ObsidianBridge.addToFmgIdIndex(fmgId, path);
+      }
+    }
+
     showMarkdownEditor.originalContent = content;
     // Update the editor to show the new frontmatter
     byId("obsidianMarkdownEditor").value = content;