feat: enable index resumption and enforce directory creation

The command now supports reopening existing indexes instead of failing when the output file exists. Control flow branches between opening an existing index and constructing a new one, moving configuration setup exclusively to the creation path. Directory existence is enforced upfront with proper I/O error propagation. The --force flag retains its original semantics by removing the target directory before proceeding with a fresh build.

DevDoc/architecture/sequences/invariant/index.html

@@ -9,7 +9,7 @@
       
       
       
-        <link rel="prev" href="../../../implementation/benchmark_query_testing/">
+        <link rel="prev" href="../../../implementation/partition_layer_cache/">
       
       
         <link rel="next" href="../../index_architecture/">
@@ -731,14 +731,14 @@
   
   
     <li class="md-nav__item">
-      <a href="../../../implementation/obilayeredmap/" class="md-nav__link">
+      <a href="../../../implementation/layer_tier/" class="md-nav__link">
         
   
   
   <span class="md-ellipsis">
     
   
-    obilayeredmap crate
+    obikindex layer tier
   
 
     
@@ -976,6 +976,34 @@
 
               
             
+              
+                
+  
+  
+  
+  
+    <li class="md-nav__item">
+      <a href="../../../implementation/partition_layer_cache/" class="md-nav__link">
+        
+  
+  
+  <span class="md-ellipsis">
+    
+  
+    Partition and layer caching (discussion)
+  
+
+    
+  </span>
+  
+  
+
+      </a>
+    </li>
+  
+
+              
+            
           </ul>
         </nav>