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/Lives.cs

@@ -0,0 +1,24 @@
+namespace Mr.BrickAdventures.Progression;
+
+/// <summary>
+/// Lives remaining in a Playthrough. Never negative.
+/// </summary>
+/// <remarks>
+/// Decrementing at zero floors rather than failing — unlike <see cref="Coins"/>, where a shortfall
+/// is a game rule the caller must handle. Flooring here is purely defensive; the invariant that one
+/// death costs exactly one life is enforced by the Attempt's outcome, not by this type.
+/// </remarks>
+public readonly struct Lives
+{
+    public int Value { get; }
+
+    private Lives(int value) => Value = value;
+
+    public static Lives Of(int value) => value < 0
+        ? throw new ArgumentOutOfRangeException(nameof(value), value, "Lives cannot be negative.")
+        : new Lives(value);
+
+    public bool IsExhausted => Value == 0;
+
+    public Lives Decrement() => new(Value == 0 ? 0 : Value - 1);
+}