feat: coins and skills move to the new domain (commit B of the swap)

atomic pair - purchasing couples them, so neither moves alone.

SkillData.Level is gone. It was an [Export] on a cached Resource: one
instance per process, so the level never persisted, leaked between save
slots, and made "unowned" indistinguishable from "level 1". Level now
comes from the Playthrough and is passed to ISkill.Initialize.

SkillManager reconciles from SkillSetChanged instead of being told by
each caller. The old shape was "record it, then tell SkillManager" as two
calls at four sites; AddSkill's early return when no player is registered
left the skill owned but inert. Reconciliation also tracks the level each
component was built at, so an upgrade rebuilds it rather than being
patched in from outside.

SkillUnlockerComponent collapses to one TryBuy - unlock and upgrade were
two methods each doing affordability, debit, level mutation and manager
notification in order.

The guard three components copy-pasted around Upgrades[Level - 1] - and
which BrickArmor omitted - now lives once in SkillComponentBase.

Hud reads coins from CoinsChanged instead of rebuilding translated
strings every frame. Lives still polls; it moves with the death path.

scripts/Resources/SkillData.cs

@@ -10,10 +10,12 @@ public partial class SkillData : Resource
     [Export] public string Name { get; set; } = "New Skill";
     [Export] public string Description { get; set; } = "New Skill";
     [Export] public Texture2D Icon { get; set; }
-    [Export] public int Level { get; set; } = 1;
     [Export] public SkillType Type { get; set; } = SkillType.Throw;
     [Export] public PackedScene Node { get; set; }
     [Export] public Array<SkillUpgrade> Upgrades { get; set; } = [];
 
+    /// <summary>
+    /// Highest level the authored upgrade table defines. Zero means this skill cannot be bought.
+    /// </summary>
     public int MaxLevel => Upgrades.Count;
 }