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/PurchaseOutcome.cs

@@ -0,0 +1,22 @@
+namespace Mr.BrickAdventures.Progression;
+
+/// <summary>
+/// Why a Skill purchase did or did not happen. Named outcomes rather than a bool, because the
+/// marketplace wants to say something different for each.
+/// </summary>
+public enum PurchaseOutcome
+{
+    Purchased,
+
+    /// <summary>The Bank plus this Attempt's coins, less what it has already spent, is not enough.</summary>
+    NotAffordable,
+
+    /// <summary>The Skill is already at the top level its authored upgrade table defines.</summary>
+    AlreadyAtTopLevel,
+
+    /// <summary>The Skill has no authored upgrades, so it cannot be bought at any price.</summary>
+    NotPurchasable,
+
+    /// <summary>There is no Playthrough loaded, or no Attempt in progress to charge it to.</summary>
+    NotPlaying,
+}