Fix Already Safe = 0: hybrid ManifestIndex preserves localIdentifiers + NAS filenames

The root cause: buildManifestIndex was discarding manifest localIdentifiers and
switching to pure filename-only matching whenever dirCount > manifest.totalCount
(which is always true when the NAS has files from other tools). If filenames didn't
match exactly, Already Safe dropped to 0 on every stale-cache reconcile.

Fix: introduce ManifestIndex(manifest:nasFilenames:totalCount:) — a hybrid that keeps
Kisani's localIdentifiers as primary keys AND adds all NAS directory filenames for
filename-fallback. Used in:
- buildManifestIndex (full reconcile)
- fast path (when dirCount > cached.entries.count, previously gated at <= 1)
- BackupEngine step 3b (so "Back up again" only uploads genuinely missing files)

Also simplify countSafe / pendingIDs / step-4 filter to always try filename fallback
(removes the isFilenameOnly gate that blocked matches when the index had IDs).

Co-Authored-By: Kutesir <kutesir@provoc.ug>
Co-Authored-By: Sentry <sentry@provoc.ug>

Core/Models/BackupManifest.swift

@@ -85,6 +85,29 @@ struct ManifestIndex: Sendable {
         self.totalCount          = names.count
     }
 
+    // Hybrid: manifest localIdentifiers (Kisani primary key) + all NAS filenames
+    // for filename-fallback matching when the NAS directory has more files than the
+    // manifest tracks (uploaded by other tools). totalCount = actual NAS file count.
+    init(manifest: BackupManifest, nasFilenames: [String], totalCount: Int) {
+        var ids = Set<String>(minimumCapacity: manifest.entries.count)
+        var names = Set<String>(minimumCapacity: manifest.entries.count + nasFilenames.count)
+        var unclaimed = Set<String>()
+        for entry in manifest.entries {
+            let lower = entry.filename.lowercased()
+            names.insert(lower)
+            if !entry.localIdentifier.isEmpty {
+                ids.insert(entry.localIdentifier)
+            } else {
+                unclaimed.insert(lower)
+            }
+        }
+        for name in nasFilenames { names.insert(name.lowercased()) }
+        self.localIdentifiers    = ids
+        self.lowercasedFilenames = names
+        self.unclaimedFilenames  = unclaimed
+        self.totalCount          = totalCount
+    }
+
     // Build from NAS directory listing (bootstrap — no localIdentifiers known)
     init(nasListing items: [NASItem]) {
         var names = Set<String>()