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/LivesTests.cs

@@ -0,0 +1,21 @@
+namespace Mr.BrickAdventures.Progression.Tests;
+
+public class LivesTests
+{
+    [Fact]
+    public void Losing_the_last_life_exhausts_them()
+    {
+        var remaining = Lives.Of(1).Decrement();
+
+        Assert.Equal(0, remaining.Value);
+        Assert.True(remaining.IsExhausted);
+    }
+
+    [Fact]
+    public void Decrementing_when_already_exhausted_stays_at_zero()
+    {
+        var remaining = Lives.Of(0).Decrement();
+
+        Assert.Equal(0, remaining.Value);
+    }
+}