e84: per-port FSM via E84PortStore

E84 (Parallel I/O) is fundamentally per-load-port: each port has its
own ten-wire handshake with the AMHS.  Earlier revisions modeled it
as a single equipment-wide FSM; this commit refactors to a per-port
store, so multi-LP tools can run independent handshakes in parallel.

Public API change in EquipmentDataModel:
  E84StateMachine e84;   -> removed
  E84PortStore    e84_ports;  // create(port_id), get(port_id), ...

Convenience pass-throughs: E84PortStore::on_signal_change auto-creates
the port on first use (ergonomic for demos); applications should call
create() explicitly with their full port set.

The two existing callsites (test_gem300_scenario, test_e87_wire_scenarios)
are updated.  The multi-LP test now demonstrates the actual win:
interleaved LP1 load + LP2 unload handshakes that reach their
respective Ready states without sequencing, and an ES on LP1 that
does NOT affect LP2 — exactly the failure mode the previous design
couldn't catch.

Five new dedicated tests in test_e84_ports.cpp for the store itself.

COMPLIANCE.md §4i updated: row now reflects per-port design.

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

include/secsgem/gem/data_model.hpp

@@ -5,6 +5,7 @@
 #include "secsgem/gem/store/cem_objects.hpp"
 #include "secsgem/gem/store/clock.hpp"
 #include "secsgem/gem/e84_state.hpp"
+#include "secsgem/gem/store/e84_ports.hpp"
 #include "secsgem/gem/ept_state.hpp"
 #include "secsgem/gem/store/control_jobs.hpp"
 #include "secsgem/gem/store/equipment_constants.hpp"
@@ -48,7 +49,7 @@ struct EquipmentDataModel {
   EptStateMachine           ept;
   CemObjectStore            cem;
   ModuleStore               modules;
-  E84StateMachine           e84;
+  E84PortStore              e84_ports;
 
   // Convenience: VID -> value lookup spanning SVIDs and DVIDs.
   std::optional<s2::Item> vid_value(uint32_t vid) const {