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/SkillIdTests.cs

@@ -0,0 +1,20 @@
+namespace Mr.BrickAdventures.Progression.Tests;
+
+public class SkillIdTests
+{
+    [Theory]
+    [InlineData("")]
+    [InlineData("   ")]
+    public void A_blank_id_is_not_a_skill(string value)
+    {
+        Assert.Throws<ArgumentException>(() => SkillId.Of(value));
+    }
+
+    [Fact]
+    public void Ids_with_the_same_text_are_the_same_skill()
+    {
+        // Owned skills are held in a dictionary keyed by id, so value equality is what makes the
+        // type usable at all rather than a convenience.
+        Assert.Equal(SkillId.Of("brick_throw"), SkillId.Of("brick_throw"));
+    }
+}