Clarify DeviceInfoProvider contract and field semantics

- Document that prop pointer must never be null
- Add @pre prop != nullptr preconditions to all accessors
- Clarify that cores_per_sm and clock_ghz are precomputed adapter
  values (not directly from cudaDeviceProp) for testability and
  metric calculations
- Document prop lifetime requirement

Fixes quality issues in Task 5 device_info_provider.cuh

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: LessUp

src/utils/device_info_provider.cuh

@@ -21,31 +21,46 @@
  *
  * Provides a minimal surface for querying GPU capabilities needed by
  * Tensor Core logic and benchmark calculations.
+ *
+ * Contract:
+ * - `prop` must never be null; all accessors dereference without null checks
+ * - `prop` must remain valid for the lifetime of this provider instance
+ * - `cores_per_sm` and `clock_ghz` are precomputed adapter values carried for
+ *   testability and metric calculations; they are NOT directly queryable from
+ *   cudaDeviceProp and must be supplied by the provider (e.g., via
+ *   DeviceInfoCache architecture tables or test fixtures)
  */
 struct DeviceInfoProvider {
     const cudaDeviceProp *prop;
     int cores_per_sm;
     float clock_ghz;
 
     /// Check if device supports Tensor Cores (sm_70+)
+    /// @pre prop != nullptr
     bool hasTensorCores() const { return prop->major >= 7; }
 
     /// Get compute capability major version
+    /// @pre prop != nullptr
     int computeMajor() const { return prop->major; }
 
     /// Get compute capability minor version
+    /// @pre prop != nullptr
     int computeMinor() const { return prop->minor; }
 
     /// Get number of streaming multiprocessors
+    /// @pre prop != nullptr
     int smCount() const { return prop->multiProcessorCount; }
 
     /// Get memory bus width in bits
+    /// @pre prop != nullptr
     int memoryBusWidth() const { return prop->memoryBusWidth; }
 
     /// Get memory clock rate in kHz
+    /// @pre prop != nullptr
     int memoryClockRate() const { return prop->memoryClockRate; }
 
     /// Get clock rate in kHz
+    /// @pre prop != nullptr
     int clockRate() const { return prop->clockRate; }
 };