feat: Sample Tag Viewer — read-only tag browser for AKAI disk images

Backend:
- extract.go: listTags() navigates to volume, runs akaiutil lstags
- extract.go: parseTags() parses tag name/index from lstags output
- serve.go: GET /api/disk/partitions — list partitions on an ISO
- serve.go: GET /api/disk/volumes — list volumes on a partition
- serve.go: GET /api/volume/tags — list tags on a volume

Web UI:
- Tag Viewer section with ISO path input and Browse button
- Partition selector buttons
- Volume grid with click-to-select
- Tag chips with color-coded dots (20-color palette)

Tests:
- 6 parseTags tests (basic, spaces, defaults, no tags, empty, no-type)
- 1 listTags integration test
- 4 API endpoint tests (partitions, missing-ISO, missing-params, tags)

Karpathy: read-only view first — no write operations yet (settagi, clrtagi)

web/ui/index.html

@@ -39,6 +39,22 @@
     <div id="extract-status"></div>
   </section>
 
+  <section id="tag-section">
+    <h2 class="section-toggle" data-section="tag-body">Tag Viewer <span class="toggle-icon">▾</span></h2>
+    <div id="tag-body">
+    <div class="tag-controls">
+      <input type="text" id="tag-iso-path" placeholder="ISO file path" value="./output/isos">
+      <button id="tag-browse-btn">Browse</button>
+    </div>
+    <div id="tag-results" style="display:none">
+      <div id="tag-status"></div>
+      <div id="tag-partitions" class="tag-partitions"></div>
+      <div id="tag-volumes" class="tag-volumes"></div>
+      <div id="tag-detail" class="tag-detail"></div>
+    </div>
+    </div>
+  </section>
+
   <section id="wav-section">
     <h2 class="section-toggle" data-section="wav-body">Sample Library <span class="toggle-icon">▾</span></h2>
     <div id="wav-body">