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/LevelTime.cs

@@ -10,7 +10,7 @@ public readonly struct LevelTime
 
     private LevelTime(double seconds) => Seconds = seconds;
 
-    public static LevelTime Of(double seconds) => seconds <= 0
-        ? throw new ArgumentOutOfRangeException(nameof(seconds), seconds, "A completion time must be positive.")
-        : new LevelTime(seconds);
+    public static LevelTime Of(double seconds) => double.IsFinite(seconds) && seconds > 0
+        ? new LevelTime(seconds)
+        : throw new ArgumentOutOfRangeException(nameof(seconds), seconds, "A completion time must be a finite positive number.");
 }