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/store/control_jobs.hpp

@@ -12,6 +12,7 @@
 #include <utility>
 #include <vector>
 
+#include "secsgem/gem/handler_slot.hpp"
 #include "secsgem/gem/control_job_state.hpp"
 
 namespace secsgem::gem {
@@ -258,7 +259,8 @@ class ControlJobStore {
 
   std::map<std::string, ControlJob> jobs_;
   TransitionTableFactory factory_;
-  StateChangeHandler on_change_;
+  HandlerSlot<const std::string&, ControlJobState, ControlJobState,
+              ControlJobEvent> on_change_;
   bool persistent_ = false;
   std::filesystem::path journal_dir_;
   uint64_t next_seq_ = 0;

include/secsgem/gem/store/process_jobs.hpp

@@ -13,6 +13,7 @@
 #include <utility>
 #include <vector>
 
+#include "secsgem/gem/handler_slot.hpp"
 #include "secsgem/gem/e40_constants.hpp"
 #include "secsgem/gem/process_job_state.hpp"
 #include "secsgem/secs2/codec.hpp"
@@ -72,7 +73,10 @@ class ProcessJobStore {
   ProcessJobStore& operator=(ProcessJobStore&&) = delete;
 
   void set_table_factory(TransitionTableFactory f) { factory_ = std::move(f); }
+  // set_ replaces the primary handler (legacy semantics); add_ appends an
+  // observer that survives set_ calls (see handler_slot.hpp).
   void set_state_change_handler(StateChangeHandler h) { on_change_ = std::move(h); }
+  void add_state_change_handler(StateChangeHandler h) { on_change_.add(std::move(h)); }
 
   enum class CreateResult {
     Created,
@@ -512,7 +516,8 @@ class ProcessJobStore {
   std::map<std::string, ProcessJob> jobs_;
   std::vector<std::string> order_;        // queue position (E40 HOQ-aware)
   TransitionTableFactory factory_;
-  StateChangeHandler on_change_;
+  HandlerSlot<const std::string&, ProcessJobState, ProcessJobState,
+              ProcessJobEvent> on_change_;
   bool persistent_ = false;
   std::filesystem::path journal_dir_;
   uint64_t next_seq_ = 0;