Fix rebucketing, seed upgrades, watch pruning, end re-stamping; add model tests

Editing a workout's start date now removes the file at its old month
bucket so the record no longer duplicates on the next reconcile. Seed
reconcile re-checks the tombstone veto before overwriting an upgraded
seed. The watch applies authoritative-empty pushes so remote deletes
prune, and a re-saved finished workout keeps its original end time.
Adds unit tests for the mappers, path bucketing, and status machine.

Claude-Session: https://claude.ai/code/session_01HJDQQDA9QdP8zByg43H5v3

Workouts/Views/Settings/SettingsView.swift

@@ -132,7 +132,8 @@ struct SettingsView: View {
                     Text("iCloud Sync")
                 }
 
-                // MARK: - Developer Section
+                // MARK: - Developer Section (debug builds only)
+                #if DEBUG
                 Section {
                     NavigationLink {
                         DuplicateCleanupView()
@@ -142,6 +143,7 @@ struct SettingsView: View {
                 } header: {
                     Text("Developer")
                 }
+                #endif
 
                 // MARK: - About Section
                 Section {