Add machine comfort settings and tighten the document schemas

Machine-based exercises now carry ordered name/value comfort settings
(seat height, back-rest position, ...) on both ExerciseDocument and, as a
plan-time snapshot, WorkoutLogDocument — the optional array doubles as the
machine flag. Editable from the exercise editor's new Machine section and
from the workout row's settings sheet, whose mid-workout edits write back
to the originating split (workout saved first, so seed clone-on-edit
repointing can't clobber the log edit).

Schema tightening rides the same rev: splits bump to v2 (weight-reminder
fields and the unused exercise category removed), workouts to v3 (the
derived `completed` flag removed; status is the single source). Starter
seeds regenerated at v2 with unchanged ULIDs; SwiftData cache schema
bumped to rebuild. SCHEMA.md documents the shapes.

Claude-Session: https://claude.ai/code/session_01LEoff8bXGBS83tK1c55Mf7

TODO.md

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+# TODO
+
+Schema work pending (being designed in a separate session) — the workout exercise
+list (`WorkoutLogListView`) has UI already waiting on these:
+
+- **Bodyweight indicator**: the row hides weight when `loadType != .weight`, treating
+  `LoadType.none` as "bodyweight". Decide whether that's the durable semantic or whether
+  exercises need an explicit `.bodyweight` load type / flag (e.g. weighted pull-ups would
+  need weight *and* bodyweight semantics).
+- **Machine comfort settings** (iOS UI built): `machineSettings: [MachineSetting]?`
+  exists on both `ExerciseDocument` and `WorkoutLogDocument` (plus the cache entities
+  and mappers). The optional doubles as the machine-based flag — nil means "not a
+  machine exercise", empty means "machine exercise, nothing recorded yet". Built on
+  iOS: the exercise add/edit screen's Machine section (toggle + ordered
+  reorderable/deletable settings via the shared `MachineSettingsEditor`), the
+  plan-time snapshot from exercise → log at workout start / add-exercise, and the
+  in-workout row's Settings affordance (shown for any machine-based log) opening an
+  editor sheet that saves to the log *and* writes back to the split's exercise as the
+  durable default (following the seed clone-on-edit redirect).
+  Remaining: **watch-side display** — the watch app doesn't yet surface or edit
+  machine settings, and its own "add exercise to a running workout" path
+  (`Workouts Watch App/Views/WorkoutLogListView.swift`) doesn't snapshot
+  `machineSettings` into the new log.