Sprint 280: Cross-language step + breakpoints (steps 1928–1932)

CrossLanguageStepOverHandler, CrossLanguageStepIntoHandler,
ASTNodeBreakpointMapper, BreakpointSyncOrchestrator — 26/26 tests pass.
Integration step 1932: Go+TypeScript poly-API round-trip verified.
Metrics: 7 whetstone calls, 5587 tokens.

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

editor/src/CrossLanguageStepIntoHandler.h

@@ -0,0 +1,36 @@
+#pragma once
+#include "PolyglotStackTraceDecoder.h"
+#include "DebugAdapterRouter.h"
+#include <string>
+#include <vector>
+
+namespace whetstone {
+
+struct StepIntoResult {
+    bool        switched         = false;
+    std::string previousLanguage;
+    std::string newLanguage;
+    int         targetFrameId    = -1;
+};
+
+class CrossLanguageStepIntoHandler {
+public:
+    // Step-into at a boundary: switch active adapter to the next frame's language.
+    StepIntoResult stepInto(const StackFrame& frame,
+                            DebugAdapterRouter& router,
+                            const std::vector<StackFrame>& frames,
+                            int frameIndex) const {
+        int next = frameIndex + 1;
+        if (!frame.isBoundary ||
+            next >= static_cast<int>(frames.size()) ||
+            frames[next].language == frame.language)
+            return {false, "", "", -1};
+
+        std::string prev    = router.getActiveLanguage();
+        std::string newLang = frames[next].language;
+        router.setActiveLanguage(newLang);
+        return {true, prev, newLang, frames[next].id};
+    }
+};
+
+} // namespace whetstone