D2: E87 wire messages (S3F17/F18 + F23/F24 + F25/F26 + F27/F28)

Adds the Carrier Management wire surface that drives the FSMs from D1:

  S3F17 / S3F18  CarrierAction (host issues ProceedWithCarrier /
                 CancelCarrier / BindCarrierID / etc.; CommandParameter
                 list is reused from S2F41).
  S3F23 / S3F24  PortGroupChangeReport (equipment notifies host of port
                 group composition changes).
  S3F25 / S3F26  CarrierTransfer (host instructs source -> target port
                 transfer).
  S3F27 / S3F28  CancelCarrier (host cancels an outstanding carrier op).

Two new ack enums in messages_helpers.hpp:
  CarrierActionAck — CAACK byte; covers the common error responses
                     (CarrierIDUnknown, Inaccessible, ActionInProgress).
  PortGroupAck     — PGACK byte (Accept / Error).

Round-trip tests for all four pairs.  Server-side dispatch + the
CarrierStore + LoadPortStore that the FSMs key into land in D3/D4.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

include/secsgem/gem/messages_helpers.hpp

@@ -171,4 +171,22 @@ enum class MapDataAck : uint8_t {  // S12F8 MAPER
   AbortNoMap   = 3,
 };
 
+// E87 §10.2 carrier-action ack code (CAACK).  Same byte used by S3F18,
+// S3F26 (carrier transfer), and S3F28 (cancel carrier).
+enum class CarrierActionAck : uint8_t {
+  Accept                  = 0,
+  Invalid                 = 1,
+  ParameterInvalid        = 2,
+  CarrierIDUnknown        = 3,
+  CarrierActionInvalid    = 4,
+  CarrierInaccessible     = 5,
+  CarrierActionInProgress = 6,
+};
+
+// E87 §10.4 port-group change ack code.
+enum class PortGroupAck : uint8_t {
+  Accept = 0,
+  Error  = 1,
+};
+
 }  // namespace secsgem::gem