J: E116 time-bucket accounting

EptStateMachine now retains per-state cumulative dwell time so the
host can read it as SVIDs.  The implementation is zero-overhead while
the FSM is idle (no timers, no background work) — on every transition
we add the prior state's dwell to its bucket and reset the entered_
timestamp.  Live dwell in the current state is included in
accumulated() via a now-vs-entered_ delta at read time.

New public API:
  accumulated(EptState)   per-state cumulative ms (incl. live dwell)
  total_elapsed()         denominator for utilization ratios
  reset_history()         S2F43-style history clear

This closes the gap I called out: previously we emitted CEIDs on
transition but didn't accumulate the bucket the host actually queries
for utilization metrics.  Wiring these into specific SVIDs is the
application's job (equipment.yaml declares SVIDs against any read
callable); the runtime data is now there.

4 new test cases cover accumulation, live-dwell inclusion, and reset.

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

include/secsgem/gem/ept_state.hpp

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <array>
 #include <chrono>
 #include <cstdint>
 #include <functional>
@@ -65,10 +66,26 @@ class EptStateMachine {
   // doesn't need a parallel timestamp tracker.
   bool on_event(EptEvent e);
 
+  // --- E116 time-bucket accounting --------------------------------------
+  // Total accumulated time spent in `s` since the FSM was constructed
+  // or since reset_history() was last called.  Time spent in the
+  // *current* state up to now is included.
+  std::chrono::milliseconds accumulated(EptState s) const;
+
+  // Total time across all states (= time since reset_history()).  Useful
+  // as the denominator for utilization / availability ratios.
+  std::chrono::milliseconds total_elapsed() const;
+
+  // S2F43-style history clear.  Sets every bucket to zero and re-anchors
+  // the current-state timer at "now".
+  void reset_history();
+
  private:
   EptTable table_;
   EptState state_ = EptState::NonScheduledTime;
   std::chrono::steady_clock::time_point entered_;
+  std::chrono::steady_clock::time_point reset_anchor_;
+  std::array<std::chrono::milliseconds, 6> buckets_{};  // indexed by EptState 0..5
   StateChangeHandler on_change_;
 };