feat(progression): Coins, LevelTime, SkillLevel core invariants (slice 1)

three bug-encoding invariants, one red/green cycle each:
- Coins.TrySubtract fails instead of clamping
- LevelTime rejects zero (continued-run zero best-times)
- SkillLevel has no zero (unowned is absence, not Level=1)

plan: drop signed NetCoins; add Attempt outcome as idempotency guard

progression/LevelTime.cs

@@ -0,0 +1,16 @@
+namespace Mr.BrickAdventures.Progression;
+
+/// <summary>
+/// A completion time for one Level. Strictly positive — a zero time means the clock was never
+/// running, which is a defect rather than a fast run.
+/// </summary>
+public readonly struct LevelTime
+{
+    public double Seconds { get; }
+
+    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);
+}