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

Shared/Model/Documents.swift

@@ -118,8 +118,13 @@ extension WorkoutDocument {
         let anyStarted = statuses.contains { $0 != .notStarted }
 
         if allResolved {
+            // Only stamp the finish time on the *transition* into completed. A workout
+            // that was already completed (with an `end`) keeps that original end, so
+            // editing a finished workout — which re-runs this recompute with all logs
+            // still resolved — doesn't jump its end forward to now.
+            let wasCompletedWithEnd = status == WorkoutStatus.completed.rawValue && end != nil
             status = WorkoutStatus.completed.rawValue
-            end = Date()
+            if !wasCompletedWithEnd { end = Date() }
         } else if anyStarted {
             status = WorkoutStatus.inProgress.rawValue
             end = nil