feat(progression): remaining slice 1 value objects

Lives (floors at zero, exhaustion observable), LevelId, SkillId,
PlaythroughId (never self-generates, keeps tests deterministic),
StatName enum, LevelTime rejects NaN/infinity.

14 tests. Arithmetic/comparison members deferred to the slices whose
behaviour needs them rather than added speculatively.

progression/PlaythroughId.cs

@@ -0,0 +1,25 @@
+namespace Mr.BrickAdventures.Progression;
+
+/// <summary>
+/// Identity of one Playthrough. Several Playthroughs exist at once and the player picks between
+/// them; "slot" is the word the UI uses for the same thing.
+/// </summary>
+/// <remarks>
+/// Deliberately has no factory that generates a value. Ids are created by an adapter and passed in,
+/// so nothing in the domain depends on a source of randomness.
+/// </remarks>
+public readonly struct PlaythroughId : IEquatable<PlaythroughId>
+{
+    public Guid Value { get; }
+
+    private PlaythroughId(Guid value) => Value = value;
+
+    public static PlaythroughId Of(Guid value) => value == Guid.Empty
+        ? throw new ArgumentException("A playthrough id cannot be the empty guid.", nameof(value))
+        : new PlaythroughId(value);
+
+    public bool Equals(PlaythroughId other) => Value.Equals(other.Value);
+    public override bool Equals(object? obj) => obj is PlaythroughId other && Equals(other);
+    public override int GetHashCode() => Value.GetHashCode();
+    public override string ToString() => Value.ToString();
+}