Sprint 281: DAP Hardening (steps 1933–1937)

DAPRequestValidator, DAPCapabilityNegotiator, DAPEventBroadcaster,
DAPBreakpointHitDispatcher — 26/26 tests pass.
Integration step 1937: Python+Rust DAP session round-trip verified.
Also adds allBreakpoints() to ASTNodeBreakpointMapper.
Metrics: 8 whetstone calls, 8853 tokens.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

editor/src/DAPEventBroadcaster.h

@@ -0,0 +1,59 @@
+#pragma once
+#include <nlohmann/json.hpp>
+#include <functional>
+#include <map>
+#include <string>
+#include <vector>
+
+namespace whetstone {
+
+class DAPEventBroadcaster {
+public:
+    using EventHandler = std::function<void(const nlohmann::json&)>;
+
+    // Subscribe to events of eventType. Returns a subscription id.
+    int subscribe(const std::string& eventType, EventHandler handler) {
+        int id = nextId_++;
+        subs_.push_back({id, eventType, std::move(handler)});
+        return id;
+    }
+
+    // Remove the subscription with the given id.
+    void unsubscribe(int id) {
+        subs_.erase(
+            std::remove_if(subs_.begin(), subs_.end(),
+                           [id](const Sub& s){ return s.id == id; }),
+            subs_.end());
+    }
+
+    // Call all handlers registered for eventType, passing body.
+    void broadcast(const std::string& eventType, const nlohmann::json& body) const {
+        for (const auto& s : subs_)
+            if (s.eventType == eventType) s.handler(body);
+    }
+
+    // Broadcast using event["event"] as the type, event itself as the body.
+    void broadcastAll(const nlohmann::json& event) const {
+        if (!event.contains("event") || !event["event"].is_string()) return;
+        broadcast(event["event"].get<std::string>(), event);
+    }
+
+    int subscriberCount(const std::string& eventType) const {
+        int n = 0;
+        for (const auto& s : subs_)
+            if (s.eventType == eventType) ++n;
+        return n;
+    }
+
+private:
+    struct Sub {
+        int          id;
+        std::string  eventType;
+        EventHandler handler;
+    };
+
+    std::vector<Sub> subs_;
+    int              nextId_ = 1;
+};
+
+} // namespace whetstone