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/StatName.cs

@@ -0,0 +1,18 @@
+namespace Mr.BrickAdventures.Progression;
+
+/// <summary>
+/// The closed set of statistics a Profile tracks. Mirrors the five names in the old
+/// <c>StatNames</c> constants, but as a type, so a typo cannot invent a sixth statistic.
+/// </summary>
+/// <remarks>
+/// Persisted names live at the persistence seam, not here — the store adapter maps these to and
+/// from the wire strings and decides what to do with unknown keys from an older save.
+/// </remarks>
+public enum StatName
+{
+    CoinsCollected,
+    EnemiesDefeated,
+    Deaths,
+    LevelsCompleted,
+    ChildrenRescued,
+}