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

Scripts/generate_starter_splits.swift

@@ -105,7 +105,7 @@ let seeds: [Sp] = [
         Ex(name: "Hollow Body Hold", sets: 3, reps: 0, load: 2, dur: 30),
         Ex(name: "Side Plank", sets: 3, reps: 0, load: 2, dur: 30),
         Ex(name: "Leg Raises", sets: 3, reps: 12, load: 0),
-        Ex(name: "Bird Dog", sets: 3, reps: 8, load: 0),
+        Ex(name: "Crunch", sets: 3, reps: 12, load: 0),
         Ex(name: "Reverse Crunch", sets: 3, reps: 12, load: 0),
     ]),
 ]