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.tests/SkillLevelTests.cs

@@ -0,0 +1,13 @@
+namespace Mr.BrickAdventures.Progression.Tests;
+
+public class SkillLevelTests
+{
+    [Fact]
+    public void There_is_no_level_zero_because_unowned_is_absence()
+    {
+        // SkillData.Level defaults to 1 today, so "not owned" and "owned at level 1" are the same
+        // value and nothing can tell them apart. A Playthrough records only the skills it owns, so
+        // absence from that record is the only way to say "unowned" — level zero must not exist.
+        Assert.Throws<ArgumentOutOfRangeException>(() => SkillLevel.Of(0));
+    }
+}