feat(progression): use cases (slice 6)

ProgressionService holds the loaded Profile, active Playthrough and
active Attempt, so "exactly one active Attempt" has a testable home.

EnterLevel always announces LevelStarted - the reason ghost recording
and splits never worked on level 1 or after a Continue. Refuses
unreachable levels. Does not touch campaign position, so a Replay
leaves it alone.

SkillCatalogue returns null for a skill with no authored upgrades,
killing the Upgrades[0].Cost overrun. PurchaseSkill reads the current
level from the ledger first, so one Attempt can unlock then upgrade.
HandlePlayerDeath now persists - death was free if you quit.

SkillLevel.First/Next driven out here by a real caller. 53 tests.

progression.tests/PlaythroughTests.cs

@@ -156,4 +156,18 @@ public class PlaythroughTests
         Assert.False(playthrough.HasCleared(First));
         Assert.Equal(First, playthrough.CurrentLevel);
     }
+
+    [Fact]
+    public void Committing_an_attempt_that_overspent_the_bank_is_an_impossible_state()
+    {
+        // PurchaseSkill's affordability check makes this unreachable through the application layer.
+        // Asserted directly so the guard is covered rather than being dead code nobody has run — if
+        // it ever fires, the bug is a missing affordability check, not this line.
+        var campaign = TwoLevels();
+        var playthrough = StartWith(Lives.Of(3));
+        var attempt = new Attempt(First);
+        attempt.TryRecordPurchase(SkillId.Of("brick_throw"), SkillLevel.First, Coins.Of(999));
+
+        Assert.Throws<InvalidOperationException>(() => playthrough.SettleCompletion(attempt, campaign));
+    }
 }