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.

Shared/Model/Documents.swift

@@ -1,4 +1,5 @@
 import Foundation
+import IndieSync
 
 /// On-disk JSON shape for each aggregate. Independent from the SwiftData cache
 /// entities so the wire format can evolve without dragging the cache schema.
@@ -29,7 +30,7 @@ struct SplitDocument: Codable, Sendable, Equatable, Identifiable {
     /// quarantining the user's whole routine.
     var activityType: Int?
 
-    static let currentSchema = 1
+    static let currentSchemaVersion = 1
 
     var relativePath: String { "Splits/\(id).json" }
 }
@@ -69,7 +70,7 @@ struct WorkoutDocument: Codable, Sendable, Equatable, Identifiable {
     // Bumped 1→2 when `metrics` was added: the captured HR/calorie data is
     // irreplaceable, so the forward-gate must quarantine these files on older apps
     // rather than let them strip `metrics` on rewrite.
-    static let currentSchema = 2
+    static let currentSchemaVersion = 2
 
     var relativePath: String { Self.relativePath(id: id, start: start) }
 
@@ -158,58 +159,7 @@ struct WorkoutMetrics: Codable, Sendable, Equatable {
 
 // MARK: - Forward-compatibility gate
 
-/// A file whose `schemaVersion` exceeds the reader's `currentSchema` was written
-/// by a newer app version; it must be quarantined, not partially decoded (Codable
-/// silently drops unknown keys) and later rewritten — which would downgrade it
-/// and lose the newer fields.
-protocol VersionedDocument {
-    var schemaVersion: Int { get }
-    static var currentSchema: Int { get }
-}
-
+// `VersionedDocument` (the `isReadable` quarantine gate for files written by a
+// newer app version), `Tombstone`, and `DocumentCoder` all live in IndieSync now.
 extension SplitDocument: VersionedDocument {}
 extension WorkoutDocument: VersionedDocument {}
-
-extension VersionedDocument {
-    /// True if this build can safely read (and rewrite) the document.
-    var isReadable: Bool { schemaVersion <= Self.currentSchema }
-}
-
-// MARK: - Soft-delete tombstone
-
-/// Lives at `Stubs/<id>.json` and tells every device "this aggregate has been
-/// deleted" — important when a remote device missed the `.removed` event for the
-/// live file because it was offline. After `gracePeriod` any device can prune it.
-struct Tombstone: Codable, Sendable, Equatable {
-    enum Kind: String, Codable, Sendable {
-        case split
-        case workout
-    }
-
-    var id: String                 // the aggregate's ULID
-    var kind: Kind
-    var deletedAt: Date
-
-    var relativePath: String { "Stubs/\(id).json" }
-
-    static let gracePeriod: TimeInterval = 30 * 24 * 60 * 60
-}
-
-// MARK: - JSON coding
-
-/// Shared encoder/decoder. ISO-8601 dates and sorted keys for human-readable,
-/// diff-friendly files when the container is browsed via the Files app.
-enum DocumentCoder {
-    static let encoder: JSONEncoder = {
-        let e = JSONEncoder()
-        e.outputFormatting = [.prettyPrinted, .sortedKeys]
-        e.dateEncodingStrategy = .iso8601
-        return e
-    }()
-
-    static let decoder: JSONDecoder = {
-        let d = JSONDecoder()
-        d.dateDecodingStrategy = .iso8601
-        return d
-    }()
-}