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/CoinsTests.cs

@@ -14,4 +14,10 @@ public class CoinsTests
         // which is the safe outcome; clamping would silently consume what wasn't there.
         Assert.Equal(Coins.Of(10), remaining);
     }
+
+    [Fact]
+    public void A_negative_quantity_of_coins_cannot_exist()
+    {
+        Assert.Throws<ArgumentOutOfRangeException>(() => Coins.Of(-1));
+    }
 }