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.tests/LevelTimeTests.cs

@@ -10,4 +10,14 @@ public class LevelTimeTests
         // files. Rejecting zero makes that unrepresentable rather than merely unlikely.
         Assert.Throws<ArgumentOutOfRangeException>(() => LevelTime.Of(0));
     }
+
+    [Theory]
+    [InlineData(double.NaN)]
+    [InlineData(double.PositiveInfinity)]
+    public void A_time_that_is_not_a_finite_number_is_rejected(double seconds)
+    {
+        // Times are derived from subtracting tick counts, so a stale or unset start time can
+        // produce NaN or infinity rather than a plausible-but-wrong number.
+        Assert.Throws<ArgumentOutOfRangeException>(() => LevelTime.Of(seconds));
+    }
 }