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/LevelTimeTests.cs

@@ -0,0 +1,13 @@
+namespace Mr.BrickAdventures.Progression.Tests;
+
+public class LevelTimeTests
+{
+    [Fact]
+    public void Zero_is_not_a_completion_time()
+    {
+        // GetCurrentLevelTime() returns 0 whenever the speedrun timer was never started, which
+        // today is every continued run — so zero-second "best times" get written to the ghost
+        // files. Rejecting zero makes that unrepresentable rather than merely unlikely.
+        Assert.Throws<ArgumentOutOfRangeException>(() => LevelTime.Of(0));
+    }
+}