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/SkillLevel.cs

@@ -0,0 +1,16 @@
+namespace Mr.BrickAdventures.Progression;
+
+/// <summary>
+/// How far a Skill has been upgraded. Starts at one; there is no zero, because a Playthrough
+/// records only the Skills it owns and absence from that record is what "unowned" means.
+/// </summary>
+public readonly struct SkillLevel
+{
+    public int Value { get; }
+
+    private SkillLevel(int value) => Value = value;
+
+    public static SkillLevel Of(int value) => value < 1
+        ? throw new ArgumentOutOfRangeException(nameof(value), value, "A skill level starts at 1; unowned is absence, not level 0.")
+        : new SkillLevel(value);
+}