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/LevelIdTests.cs

@@ -0,0 +1,14 @@
+namespace Mr.BrickAdventures.Progression.Tests;
+
+public class LevelIdTests
+{
+    [Theory]
+    [InlineData("")]
+    [InlineData("   ")]
+    public void A_blank_id_is_not_a_level(string value)
+    {
+        // Levels are addressed by a stable authored id, never by position in a list. An unset
+        // field in a .tres would otherwise become a valid id that silently collides with itself.
+        Assert.Throws<ArgumentException>(() => LevelId.Of(value));
+    }
+}