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/ProgressionEvent.cs

@@ -15,3 +15,10 @@ public sealed record RunEnded : ProgressionEvent;
 /// popup or pushes to Steam cannot fire twice however often it is told.
 /// </summary>
 public sealed record AchievementUnlocked(AchievementId Achievement) : ProgressionEvent;
+
+/// <summary>
+/// Raised whenever a Level is entered — every time, including the first Level of a new Playthrough
+/// and the first Level after resuming one. Ghost recording and the speedrun timer both start from
+/// this, so an entry path that does not announce is an entry path where neither works.
+/// </summary>
+public sealed record LevelStarted(LevelId Level) : ProgressionEvent;