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/SkillCatalogue.cs

@@ -0,0 +1,40 @@
+namespace Mr.BrickAdventures.Progression;
+
+/// <summary>
+/// The authored upgrade table for every Skill: what each level costs to reach. Built from
+/// <c>SkillData</c> resources by an adapter — same pattern as <see cref="Campaign"/>, so no port is
+/// needed and a test can construct one directly.
+/// </summary>
+public sealed class SkillCatalogue
+{
+    private readonly Dictionary<SkillId, IReadOnlyList<Coins>> _costs;
+
+    private SkillCatalogue(Dictionary<SkillId, IReadOnlyList<Coins>> costs) => _costs = costs;
+
+    public static SkillCatalogue Empty() => new(new Dictionary<SkillId, IReadOnlyList<Coins>>());
+
+    /// <summary>
+    /// Builds a catalogue. Each Skill's list is the cost to reach level 1, level 2, and so on, so a
+    /// Skill with an empty list cannot be bought at all.
+    /// </summary>
+    public static SkillCatalogue Of(IReadOnlyDictionary<SkillId, IReadOnlyList<Coins>> costsByLevel) =>
+        new(costsByLevel.ToDictionary(e => e.Key, e => e.Value));
+
+    /// <summary>
+    /// The highest level this Skill can reach, or null when it has no authored upgrades and so
+    /// cannot be bought. Returning null is what stops <c>Upgrades[0].Cost</c> from throwing on the
+    /// five Skills that currently have no upgrade entries.
+    /// </summary>
+    public SkillLevel? MaxLevelOf(SkillId skill) =>
+        _costs.TryGetValue(skill, out var costs) && costs.Count > 0
+            ? SkillLevel.Of(costs.Count)
+            : null;
+
+    /// <summary>
+    /// What it costs to reach <paramref name="level"/>, or null if this Skill does not go that high.
+    /// </summary>
+    public Coins? CostToReach(SkillId skill, SkillLevel level) =>
+        _costs.TryGetValue(skill, out var costs) && level.Value <= costs.Count
+            ? costs[level.Value - 1]
+            : null;
+}