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/SkillId.cs

@@ -0,0 +1,21 @@
+namespace Mr.BrickAdventures.Progression;
+
+/// <summary>
+/// Stable identity of a Skill, matching the authored <c>SkillData</c> name that saves already
+/// reference. Identity only — the upgrade table and presentation stay in the authored asset.
+/// </summary>
+public readonly struct SkillId : IEquatable<SkillId>
+{
+    public string Value { get; }
+
+    private SkillId(string value) => Value = value;
+
+    public static SkillId Of(string value) => string.IsNullOrWhiteSpace(value)
+        ? throw new ArgumentException("A skill id cannot be blank.", nameof(value))
+        : new SkillId(value);
+
+    public bool Equals(SkillId other) => string.Equals(Value, other.Value, StringComparison.Ordinal);
+    public override bool Equals(object? obj) => obj is SkillId other && Equals(other);
+    public override int GetHashCode() => StringComparer.Ordinal.GetHashCode(Value ?? string.Empty);
+    public override string ToString() => Value;
+}