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 Watch App/Connectivity/WatchConnectivityBridge.swift

@@ -212,7 +212,6 @@ final class WatchConnectivityBridge: NSObject {
 
     private func applyState(_ splits: [SplitDocument], workouts: [WorkoutDocument]) {
         Self.log.info("applyState: \(splits.count) splits, \(workouts.count) workouts")
-        guard !splits.isEmpty || !workouts.isEmpty else { return }
         var liveSplitIDs = Set<String>()
         for s in splits {
             CacheMapper.upsertSplit(s, relativePath: s.relativePath, into: context)