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/e84_state.hpp

@@ -0,0 +1,97 @@
+#pragma once
+
+#include <cstdint>
+#include <functional>
+#include <optional>
+#include <string>
+
+// E84 §6 Parallel I/O — the digital handshake between AMHS (Automated
+// Material Handling System) and equipment for carrier load/unload.
+//
+// E84 is signal-level, not SECS: ten parallel boolean wires between the
+// AMHS robot and the equipment, sequenced in a strict handshake.  This
+// FSM models the signal bitmap and the handshake state, accepting
+// signal-change events as input and exposing state transitions for
+// observation.  Real wiring uses opto-isolated 24V lines; we abstract
+// it as bool getters/setters so the same FSM drives both real hardware
+// and back-to-back testing.
+namespace secsgem::gem {
+
+enum class E84Signal : uint8_t {
+  CS_0   = 0,  // AMHS -> equip: carrier stage select 0
+  CS_1   = 1,  // AMHS -> equip: carrier stage select 1
+  VALID  = 2,  // AMHS -> equip: handshake start
+  TR_REQ = 3,  // AMHS -> equip: transfer request
+  BUSY   = 4,  // AMHS -> equip: transfer in progress
+  COMPT  = 5,  // AMHS -> equip: transfer complete
+  L_REQ  = 6,  // equip -> AMHS: load request (port ready to receive)
+  U_REQ  = 7,  // equip -> AMHS: unload request (port ready to release)
+  READY  = 8,  // equip -> AMHS: ready
+  ES     = 9,  // either direction: emergency stop
+};
+
+const char* e84_signal_name(E84Signal s);
+
+// 10-bit signal bitmap with bool get/set.
+class E84SignalSet {
+ public:
+  bool get(E84Signal s) const {
+    return (bits_ & (uint16_t{1} << static_cast<uint8_t>(s))) != 0;
+  }
+  void set(E84Signal s, bool v) {
+    const uint16_t mask = uint16_t{1} << static_cast<uint8_t>(s);
+    if (v) bits_ |= mask;
+    else   bits_ &= static_cast<uint16_t>(~mask);
+  }
+  uint16_t raw() const { return bits_; }
+  void clear() { bits_ = 0; }
+
+ private:
+  uint16_t bits_ = 0;
+};
+
+// E84 handoff handshake state (E84 §6.3).  Names are short for log
+// readability; semantics in comments.
+enum class E84State : uint8_t {
+  Idle           = 0,  // no signals asserted (or carrier absent)
+  CarrierPresent = 1,  // CS_0 or CS_1 asserted; no VALID yet
+  ValidAsserted  = 2,  // CS && VALID; equipment hasn't acknowledged
+  LoadReady      = 3,  // VALID && L_REQ; ready to receive carrier
+  UnloadReady    = 4,  // VALID && U_REQ; ready to release carrier
+  Transferring   = 5,  // BUSY asserted; transfer in progress
+  Complete       = 6,  // COMPT asserted; AMHS reports done
+  EmergencyStop  = 7,  // ES asserted
+  NoState        = 255,
+};
+
+const char* e84_state_name(E84State s);
+
+class E84StateMachine {
+ public:
+  using StateChangeHandler =
+      std::function<void(E84State from, E84State to, E84Signal trigger)>;
+
+  E84State        state()   const { return state_; }
+  const E84SignalSet& signals() const { return signals_; }
+  bool signal(E84Signal s) const { return signals_.get(s); }
+
+  void set_state_change_handler(StateChangeHandler h) { on_change_ = std::move(h); }
+
+  // Apply a single signal change.  Re-evaluates the handshake state
+  // and fires the change handler on transition.  Order of signal
+  // changes matters for the AMHS-equipment handshake; the FSM accepts
+  // any order and just reports the resulting state.
+  void on_signal_change(E84Signal s, bool value);
+
+  // Convenience: clear all signals; resets state to Idle.
+  void reset();
+
+ private:
+  void reevaluate(E84Signal trigger);
+
+  E84SignalSet signals_;
+  E84State state_ = E84State::Idle;
+  StateChangeHandler on_change_;
+};
+
+}  // namespace secsgem::gem