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/LevelId.cs

@@ -0,0 +1,21 @@
+namespace Mr.BrickAdventures.Progression;
+
+/// <summary>
+/// Stable identity of a Level. Never a position in a list — Playthroughs and Best Times both
+/// reference Levels, so ids must survive inserting and reordering.
+/// </summary>
+public readonly struct LevelId : IEquatable<LevelId>
+{
+    public string Value { get; }
+
+    private LevelId(string value) => Value = value;
+
+    public static LevelId Of(string value) => string.IsNullOrWhiteSpace(value)
+        ? throw new ArgumentException("A level id cannot be blank.", nameof(value))
+        : new LevelId(value);
+
+    public bool Equals(LevelId other) => string.Equals(Value, other.Value, StringComparison.Ordinal);
+    public override bool Equals(object? obj) => obj is LevelId other && Equals(other);
+    public override int GetHashCode() => StringComparer.Ordinal.GetHashCode(Value ?? string.Empty);
+    public override string ToString() => Value;
+}