Migrate the sync file layer onto the IndieSync package

Replaces the app-local copies of the extracted storage core with the
IndieSync 0.1.0 package (pinned from the new tag): ICloudFileManager ->
DocumentFileStore + TombstoneStore, ICloudFileMonitor ->
MetadataObserver (batched events with the content-date churn gate),
and the shared ULID / DocumentCoder / Tombstone / VersionedDocument
pieces. SyncEngine stays app-specific and is rewired onto the package
types; documents rename currentSchema -> currentSchemaVersion to adopt
the package protocol. ULID.make() remains as a shim minting the string
form the app keys on.

Behavior-preserving: identical JSON bytes (same coder config), same
stub wire format (kind encodes as the same strings), same soft-delete
ordering, same 30-day prune. WorkoutDocument keeps its local-calendar
month bucketing rather than adopting TimeBucketedLayout's UTC paths --
changing derivation would strand existing files. The watch target
links the package too (ULID + DocumentCoder via Shared); iOS, watchOS,
and widget targets all build.

Workouts/Seed/SplitSeeder.swift

@@ -1,3 +1,4 @@
+import IndieSync
 import Foundation
 import SwiftData
 
@@ -58,7 +59,7 @@ enum SplitSeeder {
                 )
             }
             return SplitDocument(
-                schemaVersion: SplitDocument.currentSchema, id: ULID.make(),
+                schemaVersion: SplitDocument.currentSchemaVersion, id: ULID.make(),
                 name: split.name, color: split.color, systemImage: split.icon, order: order,
                 createdAt: Date(), updatedAt: Date(), exercises: exercises,
                 activityType: split.activity.rawValue