[sensor] Use std::array in CalibratePolynomialFilter (#15264)

esphome/components/sensor/__init__.py

@@ -808,7 +808,7 @@ async def calibrate_polynomial_filter_to_code(config, filter_id):
     # Column vector
     b = [[v] for v in y]
     res = [v[0] for v in _lstsq(a, b)]
-    return cg.new_Pvariable(filter_id, res)
+    return cg.new_Pvariable(filter_id, cg.TemplateArguments(len(res)), res)
 
 
 def validate_clamp(config):

esphome/components/sensor/filter.cpp

@@ -396,14 +396,11 @@ optional<float> CalibrateLinearFilter::new_value(float value) {
   return NAN;
 }
 
-CalibratePolynomialFilter::CalibratePolynomialFilter(std::initializer_list<float> coefficients)
-    : coefficients_(coefficients) {}
-
-optional<float> CalibratePolynomialFilter::new_value(float value) {
+optional<float> calibrate_polynomial_compute(const float *coefficients, size_t count, float value) {
   float res = 0.0f;
   float x = 1.0f;
-  for (const auto &coefficient : this->coefficients_) {
-    res += x * coefficient;
+  for (size_t i = 0; i < count; i++) {
+    res += x * coefficients[i];
     x *= value;
   }
   return res;

esphome/components/sensor/filter.h

@@ -537,13 +537,21 @@ class CalibrateLinearFilter : public Filter {
   FixedVector<std::array<float, 3>> linear_functions_;
 };
 
-class CalibratePolynomialFilter : public Filter {
+/// Non-template helper for polynomial calibration (implementation in filter.cpp)
+optional<float> calibrate_polynomial_compute(const float *coefficients, size_t count, float value);
+
+/// N is set by code generation to match the exact number of polynomial coefficients.
+template<size_t N> class CalibratePolynomialFilter : public Filter {
  public:
-  explicit CalibratePolynomialFilter(std::initializer_list<float> coefficients);
-  optional<float> new_value(float value) override;
+  explicit CalibratePolynomialFilter(std::initializer_list<float> coefficients) {
+    init_array_from(this->coefficients_, coefficients);
+  }
+  optional<float> new_value(float value) override {
+    return calibrate_polynomial_compute(this->coefficients_.data(), N, value);
+  }
 
  protected:
-  FixedVector<float> coefficients_;
+  std::array<float, N> coefficients_{};
 };
 
 class ClampFilter : public Filter {