feat(domain): WordCounterService + WordCountResult + ChargeableCharConfig entity + exceptions (PRC-001)

- WordCounterService: pure domain service, 7-step algorithm (null/empty fast path → length check → emoji detection via Rune.EnumerateRunes → count specials before replace → replace specials+hyphens → collapse whitespace → tokenize)
- WordCountResult: sealed record with TotalWords + IReadOnlyDictionary<string,int> SpecialCharCounts
- 4 domain exceptions extending DomainException: EmojiDetectedException, WordCountValidationException, ChargeableCharConfigInvalidException, ChargeableCharConfigForwardOnlyException
- ChargeableCharConfig: rich entity with Create factory (invariants), Rehydrate reconstructor, ScheduleNewPrice (forward-only, returns new entity), Deactivate (idempotent)
- ChargeableCharCategories: enum-as-string constants (Currency, Percentage, Exclamation, Question, Other)
- DomainTimeProviderExtensions: internal GetArgentinaToday helper (mirrors Application.Common without creating Domain→Application dependency)
- 60 new tests: 25 golden cases all GREEN, 12 entity invariant tests, 12 exception tests, 5 WordCountResult tests, 6 ChargeableCharConfig entity tests

src/api/SIGCM2.Domain/Pricing/Exceptions/ChargeableCharConfigInvalidException.cs

@@ -0,0 +1,22 @@
+using SIGCM2.Domain.Exceptions;
+
+namespace SIGCM2.Domain.Pricing.Exceptions;
+
+/// <summary>
+/// PRC-001 — Thrown when a ChargeableCharConfig value fails domain business validation
+/// (e.g., PricePerUnit ≤ 0, Symbol empty/too long, Category unknown, ValidFrom in the past).
+/// → HTTP 400
+/// Used as defense-in-depth alongside FluentValidation in the Application layer.
+/// </summary>
+public sealed class ChargeableCharConfigInvalidException : DomainException
+{
+    public string Field { get; }
+    public string Reason { get; }
+
+    public ChargeableCharConfigInvalidException(string field, string reason)
+        : base($"Valor inválido para {field}: {reason}")
+    {
+        Field = field;
+        Reason = reason;
+    }
+}