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;
+}

progression/LevelTime.cs

@@ -10,7 +10,7 @@ public readonly struct LevelTime
 
     private LevelTime(double seconds) => Seconds = seconds;
 
-    public static LevelTime Of(double seconds) => seconds <= 0
-        ? throw new ArgumentOutOfRangeException(nameof(seconds), seconds, "A completion time must be positive.")
-        : new LevelTime(seconds);
+    public static LevelTime Of(double seconds) => double.IsFinite(seconds) && seconds > 0
+        ? new LevelTime(seconds)
+        : throw new ArgumentOutOfRangeException(nameof(seconds), seconds, "A completion time must be a finite positive number.");
 }

progression/Lives.cs

@@ -0,0 +1,24 @@
+namespace Mr.BrickAdventures.Progression;
+
+/// <summary>
+/// Lives remaining in a Playthrough. Never negative.
+/// </summary>
+/// <remarks>
+/// Decrementing at zero floors rather than failing — unlike <see cref="Coins"/>, where a shortfall
+/// is a game rule the caller must handle. Flooring here is purely defensive; the invariant that one
+/// death costs exactly one life is enforced by the Attempt's outcome, not by this type.
+/// </remarks>
+public readonly struct Lives
+{
+    public int Value { get; }
+
+    private Lives(int value) => Value = value;
+
+    public static Lives Of(int value) => value < 0
+        ? throw new ArgumentOutOfRangeException(nameof(value), value, "Lives cannot be negative.")
+        : new Lives(value);
+
+    public bool IsExhausted => Value == 0;
+
+    public Lives Decrement() => new(Value == 0 ? 0 : Value - 1);
+}

progression/PlaythroughId.cs

@@ -0,0 +1,25 @@
+namespace Mr.BrickAdventures.Progression;
+
+/// <summary>
+/// Identity of one Playthrough. Several Playthroughs exist at once and the player picks between
+/// them; "slot" is the word the UI uses for the same thing.
+/// </summary>
+/// <remarks>
+/// Deliberately has no factory that generates a value. Ids are created by an adapter and passed in,
+/// so nothing in the domain depends on a source of randomness.
+/// </remarks>
+public readonly struct PlaythroughId : IEquatable<PlaythroughId>
+{
+    public Guid Value { get; }
+
+    private PlaythroughId(Guid value) => Value = value;
+
+    public static PlaythroughId Of(Guid value) => value == Guid.Empty
+        ? throw new ArgumentException("A playthrough id cannot be the empty guid.", nameof(value))
+        : new PlaythroughId(value);
+
+    public bool Equals(PlaythroughId other) => Value.Equals(other.Value);
+    public override bool Equals(object? obj) => obj is PlaythroughId other && Equals(other);
+    public override int GetHashCode() => Value.GetHashCode();
+    public override string ToString() => Value.ToString();
+}

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;
+}

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,
+}