feat(gem): multi-observer state-change handlers via HandlerSlot

The single-slot set_*_handler pattern was a structural blocker, hit twice:
the daemon could not observe control-state changes because
register_default_handlers owns the slot, forcing GetControlState to read the
FSM cross-thread (a data race), and blocking WatchHealth and the Subscribe
stream's ControlStateChange variant.

HandlerSlot<Args...> keeps a primary slot with exact legacy semantics
(set_ replaces — one existing test depends on replacement) plus an
append-only observer list (add_) that survives set_ calls. Fire sites are
textually unchanged (operator bool / operator() / assign-from-function).

Applied to ControlStateMachine + ProcessJobStore + ControlJobStore (the
roadmap-critical three; the remaining single-slot classes follow the same
3-line pattern as needed). EquipmentRuntime gains an atomic control-state
mirror registered as an observer — control_state() is now safe from any
thread, retiring the GetControlState race — plus add_control_state_observer
and add_link_observer (selected/closed fan-out), the hooks WatchHealth and
Subscribe need.

Tests: observer ordering, set-replaces-primary-but-observers-survive,
observers-without-primary, PJ-store coexistence, and the runtime scenario
that was previously impossible (mirror + observer + default-handlers set_).
Core 464/464 (2816 assertions), daemon 16/16, live GEM300 demo passes with
single-fire control-state transitions.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

include/secsgem/gem/control_state.hpp

@@ -6,6 +6,8 @@
 #include <string>
 #include <vector>
 
+#include "secsgem/gem/handler_slot.hpp"
+
 namespace secsgem::gem {
 
 // E30 §6.2 Control State Model.
@@ -94,7 +96,10 @@ class ControlStateMachine {
   ControlState state() const { return state_; }
   bool online() const { return is_online(state_); }
 
+  // Replaces the primary handler (legacy single-slot semantics).
   void set_state_change_handler(StateChangeHandler h) { on_change_ = std::move(h); }
+  // Appends an observer that survives set_state_change_handler calls.
+  void add_state_change_handler(StateChangeHandler h) { on_change_.add(std::move(h)); }
 
   // Operator actions. Return true if a transition (or self-ack) was found.
   bool operator_online();
@@ -113,7 +118,7 @@ class ControlStateMachine {
 
   ControlTransitionTable table_;
   ControlState state_;
-  StateChangeHandler on_change_;
+  HandlerSlot<ControlState, ControlState, ControlEvent> on_change_;
 };
 
 }  // namespace secsgem::gem