Match Go NeedleMap.Delete: always write tombstone to idx file

Go's NeedleMap.Delete unconditionally writes a tombstone entry to the
idx file and updates metrics, even if the needle doesn't exist or is
already deleted. This is important for replication where every delete
operation must produce an idx write. The Rust version was skipping the
tombstone write for non-existent or already-deleted needles.

seaweed-volume/src/storage/needle_map.rs

@@ -204,24 +204,34 @@ impl CompactNeedleMap {
     }
 
     /// Mark a needle as deleted. Appends tombstone to .idx file.
-    /// Matches Go's CompactMap.Delete: checks !IsDeleted() (not IsValid()),
-    /// so needles with size==0 can still be deleted.
+    /// Matches Go's NeedleMap.Delete: ALWAYS writes tombstone to idx and
+    /// increments deletion counter, even if needle doesn't exist or is
+    /// already deleted (important for replication).
     pub fn delete(&mut self, key: NeedleId, offset: Offset) -> io::Result<Option<Size>> {
-        if let Some(old) = self.map.get(key) {
-            if !old.size.is_deleted() {
-                // Persist tombstone to idx file BEFORE mutating in-memory state for crash consistency
-                if let Some(ref mut idx_file) = self.idx_file {
-                    idx::write_index_entry(idx_file, key, offset, TOMBSTONE_FILE_SIZE)?;
-                    self.idx_file_offset += NEEDLE_MAP_ENTRY_SIZE as u64;
-                }
+        // Go unconditionally calls nm.m.Delete(), nm.logDelete(), nm.appendToIndexFile()
+        let deleted_bytes = self.map.delete(key).unwrap_or(Size(0));
+
+        // Match Go's logDelete -> LogDeletionCounter: only increment when oldSize > 0.
+        // Go does NOT decrement FileCounter/FileByteCounter in Delete;
+        // live counts are computed as FileCounter - DeletionCounter.
+        if deleted_bytes.0 > 0 {
+            self.metric.deletion_count.fetch_add(1, Ordering::Relaxed);
+            self.metric
+                .deletion_byte_count
+                .fetch_add(deleted_bytes.0 as u64, Ordering::Relaxed);
+        }
 
-                self.metric.on_delete(&old);
-                // Mark as deleted in compact map (negates size in-place)
-                self.map.delete(key);
-                return Ok(Some(old.size));
-            }
+        // Always write tombstone to idx file (matching Go)
+        if let Some(ref mut idx_file) = self.idx_file {
+            idx::write_index_entry(idx_file, key, offset, TOMBSTONE_FILE_SIZE)?;
+            self.idx_file_offset += NEEDLE_MAP_ENTRY_SIZE as u64;
+        }
+
+        if deleted_bytes.0 > 0 {
+            Ok(Some(deleted_bytes))
+        } else {
+            Ok(None)
         }
-        Ok(None)
     }
 
     // ---- Internal helpers ----