fix: use NAS directory filenames for already-safe matching when manifest is sparse

When Kisani's manifest has 0–1 entries but the NAS directory contains thousands of
files uploaded by other tools, reconcile previously showed Already Safe = 1 because
countSafe only matched against the tiny manifest.

The validity check in buildManifestIndex now:
- Stores the filtered NAS filenames in NASManifestCache (directoryFilenames field)
- Returns a ManifestIndex(nasListing:) for filename-based matching against all NAS files

The fast-path reconcile now:
- Detects a sparse manifest (entries ≤ 1, dirCount > 1)
- Rebuilds ManifestIndex(nasFilenames:) from the cached filenames for O(1) filename matching
- Falls back to full NAS reconcile if cached filenames aren't available yet

Adds ManifestIndex.init(nasFilenames:) for fast-path reconstruction from cached strings.

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

Core/Models/BackupManifest.swift

@@ -75,6 +75,16 @@ struct ManifestIndex: Sendable {
         self.totalCount = overrideTotalCount
     }
 
+    // Rebuild from cached NAS filenames (fast-path for sparse-manifest reconcile).
+    // Filenames are expected to already be filtered (no dirs, no hidden, no manifest file).
+    init(nasFilenames filenames: [String]) {
+        let names = Set(filenames.map { $0.lowercased() })
+        self.localIdentifiers   = []
+        self.lowercasedFilenames = names
+        self.unclaimedFilenames  = names
+        self.totalCount          = names.count
+    }
+
     // Build from NAS directory listing (bootstrap — no localIdentifiers known)
     init(nasListing items: [NASItem]) {
         var names = Set<String>()