docs: correct drifted and fabricated APIs in chapters 13/17/35/51

An audit of doc code blocks against the real headers found APIs that do
not exist in the codebase, presented as authoritative walkthroughs:

- ch35 (dispatch): an entirely fabricated callback architecture —
  HostCommandRegistry::set_emit_ceid_handler, CommandOutcome, emit_ceids.
  Rewritten to the real Spec/Result/dispatch + the new set_handler hook.
- ch13 (E30): wrong store names — EventStore/ReportStore -> EventReportSubscriptions,
  SvidStore -> StatusVariableStore, AlarmStore/AlarmDispatcher -> AlarmRegistry,
  ClockStore -> Clock, TerminalServiceStore -> (no store), in both the
  capability tables and the worked S2F33 example.
- ch17 (E116): EptStore/seconds/bucket_ -> EptStateMachine/milliseconds/buckets_.
- ch51 (extending): stale host-command handler -> the real set_handler signature.

Verified clean by grep: no fabricated symbols remain in docs/.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/51_extending_the_codebase.md

@@ -118,14 +118,14 @@ set-alarm, register a custom handler:
 
 ```cpp
 // At startup:
-model->commands.register_handler("VENT",
-    [model](const ParamList& params) -> CommandOutcome {
+model->commands.set_handler("VENT",
+    [model](const std::string& rcmd,
+            const std::vector<gem::CommandParameter>& params) {
       // Actually vent the chamber here.
-      if (!vacuum_safe_to_vent()) {
-        return {HostCmdAck::CannotPerformNow, {}};
-      }
+      if (!vacuum_safe_to_vent())
+        return gem::HostCmdAck::CannotDoNow;
       hardware_vent_chamber();
-      return {HostCmdAck::Accept, {}};
+      return gem::HostCmdAck::Accept;
     });
 ```