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/PlaythroughIdTests.cs

@@ -0,0 +1,13 @@
+namespace Mr.BrickAdventures.Progression.Tests;
+
+public class PlaythroughIdTests
+{
+    [Fact]
+    public void The_empty_guid_is_not_a_playthrough()
+    {
+        // Ids arrive from an adapter, never from Guid.NewGuid() inside the domain — otherwise every
+        // test touching identity would be non-deterministic. That makes a default-valued guid a
+        // realistic mistake worth refusing.
+        Assert.Throws<ArgumentException>(() => PlaythroughId.Of(Guid.Empty));
+    }
+}