fix: parameterize caller-saved threshold for GP/FP in ColorGraph

Address code review feedback:
- Add caller_saved_threshold parameter to ColorGraph (GP: 19, FP: 16)
- Replace std::vector heap allocation with two-pass scan for zero overhead
- Fix semantic error: c<19 was incorrect for FP (s16-s18 are callee-saved)

src/mir/RegAlloc.cpp

@@ -599,7 +599,8 @@ namespace mir
     static GraphColoringResult ColorGraph(
         InterferenceGraph &graph,
         const std::vector<int> &allocatable_regs,
-        MachineFunction & /*function*/)
+        MachineFunction & /*function*/,
+        int caller_saved_threshold = 19)
     {
       const int K = static_cast<int>(allocatable_regs.size());
       GraphColoringResult result;
@@ -726,24 +727,29 @@ namespace mir
         }
 
         int assigned_color = -1;
-        // 收集可用颜色，区分caller-saved和callee-saved
-        std::vector<int> caller_saved_avail;
-        std::vector<int> callee_saved_avail;
+        // 第一遍：优先选 caller-saved 颜色 (c < caller_saved_threshold)
         for (int c : allocatable_regs)
         {
+          if (c >= caller_saved_threshold) break;
           if (used_colors.find(c) == used_colors.end())
           {
-            if (c < 19)
-              caller_saved_avail.push_back(c);
-            else
-              callee_saved_avail.push_back(c);
+            assigned_color = c;
+            break;
+          }
+        }
+        // 第二遍：若 caller-saved 无可用，选 callee-saved
+        if (assigned_color < 0)
+        {
+          for (int c : allocatable_regs)
+          {
+            if (c < caller_saved_threshold) continue;
+            if (used_colors.find(c) == used_colors.end())
+            {
+              assigned_color = c;
+              break;
+            }
           }
         }
-        // 优先选择caller-saved颜色
-        if (!caller_saved_avail.empty())
-          assigned_color = caller_saved_avail[0];
-        else if (!callee_saved_avail.empty())
-          assigned_color = callee_saved_avail[0];
 
         if (assigned_color >= 0)
         {
@@ -996,8 +1002,8 @@ namespace mir
         BuildInterferenceForGP(function, block_liveness, gp_alloc, gp_graph);
         BuildInterferenceForFP(function, block_liveness, fp_alloc, fp_graph);
 
-        auto gp_result = ColorGraph(gp_graph, gp_alloc, function);
-        auto fp_result = ColorGraph(fp_graph, fp_alloc, function);
+        auto gp_result = ColorGraph(gp_graph, gp_alloc, function, 19);
+        auto fp_result = ColorGraph(fp_graph, fp_alloc, function, 16);
 
         if (gp_result.spilled.empty() && fp_result.spilled.empty())
         {