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/Playthrough.cs

@@ -81,12 +81,16 @@ public sealed class Playthrough
     /// if it is a Replay. Decides which from the Attempt's own Level, so a Replay cannot advance
     /// campaign position (ADR-0003). Does nothing if the Attempt has already ended.
     /// </summary>
-    public void SettleCompletion(Attempt attempt, Campaign campaign)
+    /// <returns>
+    /// True when this was a First Clear and the ledger was committed. False for a Replay, or when the
+    /// Attempt had already ended — in both cases nothing here changed.
+    /// </returns>
+    public bool SettleCompletion(Attempt attempt, Campaign campaign)
     {
         var isFirstClear = !HasCleared(attempt.Level);
 
-        if (!attempt.TryEndInCompletion()) return;
-        if (!isFirstClear) return;
+        if (!attempt.TryEndInCompletion()) return false;
+        if (!isFirstClear) return false;
 
         if (!Bank.Add(attempt.CoinsFound).TrySubtract(attempt.CoinsSpent, out var settled))
             throw new InvalidOperationException(
@@ -101,20 +105,25 @@ public sealed class Playthrough
 
         if (campaign.Next(attempt.Level) is { } next)
             CurrentLevel = next;
+
+        return true;
     }
 
     /// <summary>
     /// Ends the given Attempt in death and charges one life for it. Does nothing if the Attempt has
     /// already ended, so a repeated death event costs no further lives.
     /// </summary>
-    public void SettleDeath(Attempt attempt)
+    /// <returns>False if the Attempt had already ended, in which case nothing changed.</returns>
+    public bool SettleDeath(Attempt attempt)
     {
-        if (!attempt.TryEndInDeath()) return;
+        if (!attempt.TryEndInDeath()) return false;
 
         Lives = Lives.Decrement();
 
         if (Lives.IsExhausted)
             _events.Raise(new RunEnded());
+
+        return true;
     }
 
     /// <summary>