AP_ExternalAHRS_InertialSense: remove SDK duplication, use ArduPilot logging

- Move ISB data-set types (DID constants, status enums, payload structs,
  RMC macros) from the header into the .cpp; forward-declare payload types
  in the header so the class interface remains intact.
- Replace IS_PUSH_PACK / IS_POP_PACK / IS_PACKED with #pragma pack.
- Remove duplicated utilities: _MIN macro (use MIN()), move_buffer_32bit()
  (use memmove()), MEMCPY_INC macro (inlined).
- Rename is_comm_fletcher16 -> isb_fletcher16 and drop the
  is_comm_isb_checksum16 alias; add comment explaining why it cannot be
  replaced by AP_Math crc_fletcher16() (different modulo semantics).
- Replace all printf() calls with GCS_SEND_TEXT at appropriate MAVLink
  severities (INFO / WARNING / CRITICAL / ERROR).  Transition-edge gating
  in handleIns3Message prevents GCS flooding at the INS_3 message rate.
- Remove hal.scheduler->delay(1000) calls from the constructor.
- Remove now-unused #include <stdio.h> and #include <algorithm>.
- Baudrate: use sm.find_baudrate() instead of hardcoded 921600
- Replace #pragma pack(push,1)/#pragma pack(pop) with PACKED macro on
  individual structs, matching ArduPilot convention
- pre_arm_check: remove trailing \n from failure message
- Remove malformed commented-out IMU request block
- Initialize vel_cov/pos_cov/hgt_cov to 0 to prevent garbage returns
  from get_variances() before first DID_INL2_NED_SIGMA message
- Remove unused members port_open and ppd_fd
- Thread name: "AHRS" -> "IS" to avoid collision with VectorNav
- Magnetometer: remove device-specific SN510457 scaling; pass
  normalized values and let ArduPilot calibration handle scaling
- INS_3 periodMultiple: use MAX(1, 250/get_rate()) to respect EAHRS_RATE
- Barometer: 1e3 -> 1000.0f (avoid double arithmetic)
- Remove redundant is_comm_enable_protocol() call (already set by
  is_comm_register_isb_handler)
- Remove copy-pasted "GPS at 200ms" comments from vel/RTK requests
- Remove unsubscribed IMU path: handlePimuMessage, pimu_t, DID_PIMU,
  imu_sample_duration, last_imu_pkt
- Replace typedef enum/typedef struct with plain enum/struct (C++ style)
- Add space after if/while/switch keywords throughout
- Fix Allman-style braces to K&R (opening brace on same line) in
  get_filter_status(), initialize(), handleIns3Message()
- Add braces to all single-line if statements
- Remove unused includes: AP_AHRS, AP_Filesystem, AP_InertialSensor,
  AP_Logger, AP_HAL/utility/sparse-endian, cstdint
- Align parameter names between declarations and definitions
- Implement num_gps_sensors()
- Allman braces on all function definitions
- camelCase methods renamed to snake_case (handle_*, parse_isb_data, etc.)
- typedef → using for all type aliases (port_handle_t, function pointers,
  eDataIDs)
- typedef union → union without typedef
- Enum names renamed to PascalCase (ProtocolType, ParseErrorType,
  GpsNavFixStatus, InsStatusFlags, PktSpecialChars, etc.)
- Space before colon in class declaration

Author: oursland