Guard NavigationLink destination inits against deleted @Models

A closure-based NavigationLink builds its destination view eagerly for
every row on every parent-list body evaluation — including the view-graph
update SwiftUI runs the instant SyncEngine.delete does context.delete +
save. Mapping the row's just-deleted @Model to a document there reads a
persisted property on a dead model, which traps in SwiftData
(_InitialBackingData.getValue -> assertionFailure). This is the "Crashed
when deleted a workout" TestFlight report on 2.3 (124).

Add WorkoutDocument(fromLive:) (nil when isDeleted) + a .deletedPlaceholder
and route every destination-init / updatedAt-absorb map through it:
phone WorkoutLogListView + SplitDetailView (read split.id) and watch
WorkoutLogListView. Fixes the whole crash class, not just the report.

Claude-Session: https://claude.ai/code/session_01BQcEWmAPA78338QuEwRkAh

Shared/Model/Mappers.swift

@@ -54,6 +54,30 @@ extension WorkoutDocument {
                   deletedLogIDs: workout.deletedLogIDs,
                   restSeconds: workout.restSeconds, autoAdvance: workout.autoAdvance)
     }
+
+    /// Maps a *live* cache entity to a document, or `nil` when that entity has already
+    /// been deleted from its SwiftData context. Reading a persisted property on a
+    /// deleted `@Model` traps (`_InitialBackingData.getValue` → `assertionFailure`), and
+    /// a closure-based `NavigationLink` builds its destination view — and so runs this
+    /// map — eagerly for *every* row in the parent list, including during the view-graph
+    /// update that fires the instant a workout is deleted. Any map of an entity not
+    /// already known to be live must go through this guard.
+    init?(fromLive workout: Workout) {
+        guard !workout.isDeleted else { return nil }
+        self.init(from: workout)
+    }
+
+    /// An inert stand-in for a destination whose backing entity is gone: the row is
+    /// being removed, so its detail screen is never actually shown. Reads nothing from
+    /// any entity, so it is safe to substitute for a deleted `@Model`.
+    static let deletedPlaceholder = WorkoutDocument(
+        schemaVersion: WorkoutDocument.currentSchemaVersion,
+        id: "",
+        start: Date(timeIntervalSinceReferenceDate: 0),
+        status: WorkoutStatus.notStarted.rawValue,
+        createdAt: Date(timeIntervalSinceReferenceDate: 0),
+        updatedAt: Date(timeIntervalSinceReferenceDate: 0),
+        logs: [])
 }
 
 // MARK: - Document → Cache (upsert)