feat(thermal): certify exact binary64 time grids

include/fesa/analysis/thermal_time_grid.h

@@ -1,11 +1,44 @@
 #ifndef FESA_ANALYSIS_THERMAL_TIME_GRID_H_
 #define FESA_ANALYSIS_THERMAL_TIME_GRID_H_
+
 #include <cstdint>
+
 #include "fesa/core/status.h"
+
 namespace fesa {
-class ThermalTimeGrid final { public:
- static Result<ThermalTimeGrid> Create(double period,double increment,std::uint32_t maximum);
- std::uint32_t Count() const noexcept; double Time(std::uint32_t index) const noexcept; double Delta(std::uint32_t index) const noexcept;
- private: ThermalTimeGrid(double p,double d,std::uint32_t n,bool close):period_(p),increment_(d),count_(n),close_(close){} double period_,increment_; std::uint32_t count_; bool close_; };
-}
-#endif
+
+/// @brief Carries one stored time and its actual integration coefficient.
+struct ThermalTimeFrame {
+  double time;
+  double delta;
+};
+
+/// @brief Stores an O(1) fixed-increment grid with exact input-cell endpoint tests.
+class ThermalTimeGrid final {
+ public:
+  /// @brief Checks finite positive controls and the uint32 INC bound before use.
+  static Result<ThermalTimeGrid> Create(double period, double increment,
+                                       std::uint32_t maximum = 100U);
+  /// @brief Returns the number of completed increments, excluding initial time.
+  std::uint32_t Count() const noexcept;
+  /// @brief Returns fl(index * nominal increment), or the exact final input time.
+  /// @pre index is in [0, Count()].
+  double Time(std::uint32_t index) const noexcept;
+  /// @brief Returns the actual coefficient, never a difference of stored times.
+  /// @pre index is in [1, Count()].
+  double Delta(std::uint32_t index) const noexcept;
+  /// @brief Checks index, finite values and strict increase for one frame in O(1).
+  Result<ThermalTimeFrame> Frame(std::uint32_t index) const;
+
+ private:
+  /// @brief Stores controls only after count and terminal interval validation.
+  ThermalTimeGrid(double period, double increment, std::uint32_t count,
+                  double final_delta);
+  double period_;
+  double increment_;
+  std::uint32_t count_;
+  double final_delta_;
+};
+
+}  // namespace fesa
+#endif  // FESA_ANALYSIS_THERMAL_TIME_GRID_H_