[pruthvi828]

Feature

This PR adds first-class support for Remote Transmission Request (RTR) frames to CanMsg, closing #230.

Background

The CAN standard defines the RTR (Remote Transmission Request) bit in arbitration headers to differentiate remote request frames from data frames. Under Linux SocketCAN (linux/can.h), which CanMsg.h explicitly references in its comment block (lines 127–132):

• Bit 31: Frame format flag (CAN_EFF_FLAG = 0x80000000U)
• Bit 30: Remote transmission request flag (CAN_RTR_FLAG = 0x40000000U)

Prior to this PR, bit 30 was marked as /* reserved (future remote transmission request flag) */, but lacked definition and API helpers, preventing Arduino platforms (such as Renesas Uno R4 Minima/WiFi, ESP32, STM32) from representing or checking for RTR frames.

Changes

1. api/CanMsg.h:

   • Defined static uint32_t constexpr CAN_RTR_FLAG = 0x40000000U;.
   • Updated constructor CanMsg(can_id, len, ptr, bool const rtr = false) with default argument rtr = false ensuring 100% backward compatibility with all existing code.
   • Added query methods:
     • bool isRtr() const: Returns true if CAN_RTR_FLAG is set.
     • bool isRemote() const: Alias matching isRtr().
   • Added free helper functions:
     • CanStandardRtrId(uint32_t const id): Constructs a standard 11-bit ID with RTR flag set.
     • CanExtendedRtrId(uint32_t const id): Constructs an extended 29-bit ID with EFF and RTR flags set.
   • Updated printTo(Print & p) to emit RTR indicator for remote request frames ([020 RTR] (0) : ) while leaving standard data frame output identical.

2. Tests (test/src/CanMsg/):

   • Added test_isRtr.cpp with Catch2 test cases covering standard RTR, extended RTR, constructor flag, and false assertions on standard data frames.
   • Extended test_printTo.cpp with test cases for standard RTR and extended RTR frame serialization.
   • Registered test_isRtr.cpp in test/CMakeLists.txt.

Verification

• Syntax and header validation verified with LLVM clang++ (-std=c++11 -Wall -Wextra).
• All existing tests in test/src/CanMsg/ retain identical behavior and assertions.

Fixes #230