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/CoinsTests.cs

@@ -0,0 +1,17 @@
+namespace Mr.BrickAdventures.Progression.Tests;
+
+public class CoinsTests
+{
+    [Fact]
+    public void Subtracting_more_than_held_fails_and_leaves_the_amount_untouched()
+    {
+        var held = Coins.Of(10);
+
+        var succeeded = held.TrySubtract(Coins.Of(30), out var remaining);
+
+        Assert.False(succeeded);
+        // Not clamped to zero. A caller that ignores the bool sees coins un-spent,
+        // which is the safe outcome; clamping would silently consume what wasn't there.
+        Assert.Equal(Coins.Of(10), remaining);
+    }
+}