refactor(core): share quantization math and reuse residuals

include/rabitqlib/core/detail/quantized_level.hpp

@@ -0,0 +1,23 @@
+#pragma once
+
+#include <algorithm>
+
+namespace rabitqlib::quant::detail {
+
+// Preconditions: magnitude >= 0 and finite; scale >= 0 and finite; max_code >= 0.
+// Keep this helper local to each translation unit: callers use different ISA flags.
+// Search evaluation and final encoding must use the same division-based thresholds.
+static inline int quantized_level_at_scale(double magnitude, double scale, int max_code) {
+    if (magnitude == 0)
+        return 0;
+    int code = static_cast<int>(std::min(scale * magnitude, static_cast<double>(max_code)));
+    // Multiplication can round to either side of an event; correct against the
+    // exact thresholds rather than adding an epsilon that could cross other events.
+    if (code < max_code && (code + 1.0) / magnitude <= scale)
+        ++code;
+    else if (code > 0 && static_cast<double>(code) / magnitude > scale)
+        --code;
+    return code;
+}
+
+}  // namespace rabitqlib::quant::detail