feat(progression): Playthrough aggregate (slice 3)

no LoseLife (ADR-0005). SettleDeath / SettleCompletion defer to the
Attempt's terminal outcome, so both are atomic and idempotent.
SettleCompletion branches on HasCleared itself - a Replay cannot bank
or move campaign position backwards (ADR-0003).

Campaign value object holds authored level order; ReachableLevels is
derived, replacing the stored UnlockedLevels. Coins.IsAtLeast driven
out by cross-scope affordability.

29 tests. CONTEXT.md: Campaign, Settle.

progression/Campaign.cs

@@ -0,0 +1,37 @@
+namespace Mr.BrickAdventures.Progression;
+
+/// <summary>
+/// The authored ordering of Levels that defines progression. Shared by every Playthrough — it is
+/// authored data, not per-Playthrough state, and carries no scene or presentation detail.
+/// </summary>
+public sealed class Campaign
+{
+    private readonly List<LevelId> _levels;
+
+    private Campaign(List<LevelId> levels) => _levels = levels;
+
+    public static Campaign Of(IEnumerable<LevelId> levels)
+    {
+        var ordered = levels.ToList();
+
+        if (ordered.Count == 0)
+            throw new ArgumentException("A campaign needs at least one level.", nameof(levels));
+
+        if (ordered.Distinct().Count() != ordered.Count)
+            throw new ArgumentException("A campaign cannot list the same level twice.", nameof(levels));
+
+        return new Campaign(ordered);
+    }
+
+    public LevelId First => _levels[0];
+
+    /// <summary>
+    /// The Level after <paramref name="level"/>, or null if it is the last one or not in this
+    /// Campaign at all.
+    /// </summary>
+    public LevelId? Next(LevelId level)
+    {
+        var at = _levels.IndexOf(level);
+        return at >= 0 && at + 1 < _levels.Count ? _levels[at + 1] : null;
+    }
+}