feat: add absorb command — merge source entity into target

DB: Absorb() merges body (newline-separated), unions tags, demotes
crystallized sources, soft-deletes source. Rejects crystallized targets.

API: POST /api/entities/:id/absorb { source_id }

CLI: nib absorb <target> <source> with prefix ID resolution

Web: absorb button on fluid entities, 'a' keyboard shortcut,
source picker modal

web/index.html

@@ -59,6 +59,15 @@
         </div>
     </div>
 
+    <div id="absorb-modal" class="modal hidden">
+        <div class="modal-backdrop"></div>
+        <div class="modal-content">
+            <h3>absorb source into target</h3>
+            <div id="absorb-source-list" class="absorb-list"></div>
+            <button class="modal-close">esc to cancel</button>
+        </div>
+    </div>
+
     <script src="/app.js"></script>
 </body>
 </html>