Sprint 283: poly-pipeline (steps 1943–1947)

PipelineStageSpec/Registry, PipelineGraph (Kahn topo sort), PipelineExecutionPlan,
PipelineParityValidator — 26/26 tests pass.
Integration step 1947: Python→Rust→Go→Haskell→TypeScript pipeline verified.
Metrics: 7 whetstone calls, 8566 tokens.

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

editor/src/PipelineExecutionPlan.h

@@ -0,0 +1,56 @@
+#pragma once
+#include "PipelineGraph.h"
+#include "PipelineStageSpec.h"
+#include <string>
+#include <vector>
+
+namespace whetstone {
+
+class PipelineExecutionPlan {
+public:
+    void addStage(const PipelineStageSpec& spec) {
+        registry_.registerStage(spec);
+        graph_.addStage(spec.stageId);
+    }
+
+    void addEdge(const std::string& fromId, const std::string& toId) {
+        graph_.addEdge(fromId, toId);
+    }
+
+    // Stages in topological order.
+    std::vector<PipelineStageSpec> executionOrder() const {
+        std::vector<PipelineStageSpec> result;
+        for (const auto& id : graph_.topologicalOrder())
+            result.push_back(registry_.getStage(id));
+        return result;
+    }
+
+    // Language per stage in execution order.
+    std::vector<std::string> languageSequence() const {
+        std::vector<std::string> langs;
+        for (const auto& spec : executionOrder())
+            langs.push_back(spec.language);
+        return langs;
+    }
+
+    bool isValid() const {
+        return !graph_.hasCycle() && graph_.stageCount() > 0;
+    }
+
+    // Pairs "langA->langB" wherever adjacent stages in execution order differ in language.
+    std::vector<std::string> boundaryPairs() const {
+        auto order = executionOrder();
+        std::vector<std::string> pairs;
+        for (std::size_t i = 1; i < order.size(); ++i) {
+            if (order[i-1].language != order[i].language)
+                pairs.push_back(order[i-1].language + "->" + order[i].language);
+        }
+        return pairs;
+    }
+
+private:
+    PipelineStageRegistry registry_;
+    PipelineGraph         graph_;
+};
+
+} // namespace whetstone