P: E84 Parallel I/O handoff signaling

The biggest single gap I called out in the GEM300 audit — closed.
E84 is the digital handshake between AMHS (Automated Material
Handling System) and the equipment for carrier load/unload.  Unlike
the rest of GEM300, this isn't SECS messaging; it's a fixed set of
ten parallel boolean wires that follow a strict sequencing protocol
(E84-0710 §6.3).

Adds:
  E84Signal enum     CS_0/CS_1/VALID/TR_REQ/BUSY/COMPT/L_REQ/U_REQ/
                     READY/ES
  E84SignalSet       10-bit bitmap with bool get/set
  E84State           Idle / CarrierPresent / ValidAsserted /
                     LoadReady / UnloadReady / Transferring /
                     Complete / EmergencyStop
  E84StateMachine    re-evaluates state on every signal change,
                     observable via set_state_change_handler

Joins EquipmentDataModel as `e84` (top-level — there's one per tool,
not per port).  ES (emergency stop) dominates regardless of other
signals; COMPT and BUSY override the VALID-handshake states.  Same
FSM drives real opto-isolated I/O lines (when wired through an
asio digital input adapter) and the back-to-back test simulation.

Six test cases cover the full load handshake trace (six transitions,
including the transient LoadReady-after-BUSY-drops state), the
unload variant via U_REQ, ES dominance + recovery, reset(), and
no-op suppression for idempotent signal writes.

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

include/secsgem/gem/data_model.hpp

@@ -4,6 +4,7 @@
 #include "secsgem/gem/store/carriers.hpp"
 #include "secsgem/gem/store/cem_objects.hpp"
 #include "secsgem/gem/store/clock.hpp"
+#include "secsgem/gem/e84_state.hpp"
 #include "secsgem/gem/ept_state.hpp"
 #include "secsgem/gem/store/control_jobs.hpp"
 #include "secsgem/gem/store/equipment_constants.hpp"
@@ -47,6 +48,7 @@ struct EquipmentDataModel {
   EptStateMachine           ept;
   CemObjectStore            cem;
   ModuleStore               modules;
+  E84StateMachine           e84;
 
   // Convenience: VID -> value lookup spanning SVIDs and DVIDs.
   std::optional<s2::Item> vid_value(uint32_t vid) const {