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/implementation/pipeline.refs/index.html

@@ -727,14 +727,14 @@
   
   
     <li class="md-nav__item">
-      <a href="../obilayeredmap/" class="md-nav__link">
+      <a href="../layer_tier/" class="md-nav__link">
         
   
   
   <span class="md-ellipsis">
     
   
-    obilayeredmap crate
+    obikindex layer tier
   
 
     
@@ -972,6 +972,34 @@
 
               
             
+              
+                
+  
+  
+  
+  
+    <li class="md-nav__item">
+      <a href="../partition_layer_cache/" class="md-nav__link">
+        
+  
+  
+  <span class="md-ellipsis">
+    
+  
+    Partition and layer caching (discussion)
+  
+
+    
+  </span>
+  
+  
+
+      </a>
+    </li>
+  
+
+              
+            
           </ul>
         </nav>
       
@@ -1242,12 +1270,12 @@
 <h1 id="coverage-implementationpipelinemd">Coverage: implementation/pipeline.md</h1>
 <h2 id="code-couvert">Code couvert</h2>
 <ul>
-<li><code>obikpartitionner/src/partition.rs</code> — estimation des paramètres (phase 0)</li>
+<li><code>obikindex/src/partition/partition.rs</code> — estimation des paramètres (phase 0)</li>
 <li><code>obiskbuilder/src/iter.rs</code> — scatter : filtre entropie, extraction superkmers, routage partition (phase 1)</li>
-<li><code>obikpartitionner/src/filter.rs</code> — déduplication bucket-sort (phase 2)</li>
-<li><code>obikpartitionner/src/kmer_sort.rs</code> — tri externe + agrégation de comptages (phase 3)</li>
+<li><code>obikindex/src/partition/filter.rs</code> — déduplication bucket-sort (phase 2)</li>
+<li><code>obikindex/src/partition/kmer_sort.rs</code> — tri externe + agrégation de comptages (phase 3)</li>
 <li><code>obidebruinj/src/debruijn.rs</code> — graphe De Bruijn, extraction des unitigs (phase 5)</li>
-<li><code>obikpartitionner/src/index_layer.rs</code> — construction MPHF + évidence (phase 6), paramètre <code>block_bits</code></li>
+<li><code>obikindex/src/partition/index_layer.rs</code> — construction MPHF + évidence (phase 6), paramètre <code>block_bits</code></li>
 <li><code>obikindex/src/index.rs</code> — <code>build_layers()</code>, <code>dereplicate_and_count()</code></li>
 </ul>
 <h2 id="notes">Notes</h2>