feat(progression): Attempt ledger with terminal-outcome guard (slice 2)

Attempt carries its LevelId so settlement decides First Clear vs Replay
without trusting the caller. Outcome is terminal and refuses to change:
one death costs one life, one completion banks once.

Coins.Zero/Add driven out by the ledger rather than added up front.
21 tests. ADR-0005: Playthrough settles Attempts, has no LoseLife.

progression/Coins.cs

@@ -10,10 +10,14 @@ public readonly struct Coins : IEquatable<Coins>
 
     private Coins(int value) => Value = value;
 
+    public static readonly Coins Zero = new(0);
+
     public static Coins Of(int value) => value < 0
         ? throw new ArgumentOutOfRangeException(nameof(value), value, "Coins cannot be negative.")
         : new Coins(value);
 
+    public Coins Add(Coins other) => new(Value + other.Value);
+
     /// <summary>
     /// Subtracts <paramref name="amount"/>. Returns false when the amount exceeds what is held,
     /// leaving <paramref name="remaining"/> untouched rather than clamping to zero.