Sprint 64: Cost-Aware Transpilation Planning — Steps 869-878

Adds cost modeling and budget enforcement for transpilation runs:
- PortingCostModel: LOC + ambiguity → compute/review/total cost + tier (step 869)
- ReviewEffortEstimator: ambiguity/risk → minutes + complexity label (step 870)
- ComputeCostEstimator: prior run history → avg duration/tokens (step 871)
- MultiPlanAlternativeGenerator: cheap/balanced/rigorous profiles (step 872)
- BudgetPolicyEnforcer: approve/requires_token/rejected decisions (step 873)
- CostQualityTradeoffReport: efficiency-based plan recommendation (step 874)
- whetstone_plan_transpilation_run MCP tool (step 875)
- whetstone_estimate_porting_cost MCP tool (step 876)
- CostTelemetryIntegration: cost error tracking + over-budget count (step 877)
- Sprint64IntegrationSummary (step 878)

All 10 steps passing (94 tests). Policy: over-budget plans require
explicit reviewer approval token.

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

editor/src/graduation/PortingCostModel.h

@@ -0,0 +1,34 @@
+#pragma once
+// Step 869: Porting cost model v1.
+#include <string>
+#include <nlohmann/json.hpp>
+
+struct PortingCostEstimate {
+    std::string pairId;
+    float computeCost = 0.0f;   // normalized 0-1
+    float reviewCost = 0.0f;    // normalized 0-1
+    float totalCost = 0.0f;
+    std::string tier;           // "low", "medium", "high"
+};
+
+class PortingCostModel {
+public:
+    static PortingCostEstimate estimate(const std::string& pairId,
+                                        int linesOfCode, float ambiguityScore) {
+        PortingCostEstimate e;
+        e.pairId = pairId;
+        e.computeCost = std::min(1.0f, linesOfCode / 1000.0f);
+        e.reviewCost = std::min(1.0f, ambiguityScore);
+        e.totalCost = (e.computeCost + e.reviewCost) / 2.0f;
+        if (e.totalCost < 0.33f)      e.tier = "low";
+        else if (e.totalCost < 0.66f) e.tier = "medium";
+        else                          e.tier = "high";
+        return e;
+    }
+
+    static nlohmann::json toJson(const PortingCostEstimate& e) {
+        return {{"pair_id", e.pairId}, {"compute_cost", e.computeCost},
+                {"review_cost", e.reviewCost}, {"total_cost", e.totalCost},
+                {"tier", e.tier}};
+    }
+};