Initial public release of WII5 Buoy firmware

Firmware for an autonomous wave-measurement buoy (ATmega2560-based
WII5 v2 board). Reads wave motion from a Sparton AHRS-M1/M2 IMU,
samples GPS and battery state, and reports back over Iridium SBD
satellite telemetry. Originally developed 2012-2024.

This is the first public release. Code, documentation, and field-tested
operating modes (Capture, Sleep, Position, ManualTest, SelfTest,
LowBattery) are licensed under Apache 2.0 — see LICENSE and NOTICE.

See README.md for an overview and build instructions, CONTRIBUTING.md
for how to contribute, and DEPLOYMENTS.md for the field-deployment log.

doc/EXTERNAL_REFERENCES.md

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+# External References
+
+Vendor documentation that the WII5 buoy firmware was developed against.
+These documents are not redistributed in this repository (their licensing is
+unclear); refer to the vendor sites for current copies.
+
+## Iridium 9602 / 9603 SBD modems
+
+Used for satellite telemetry. Key reference is the **Iridium 9602 / 9603
+SBD Transceiver Developer's Guide** (AT-command set, SBD message lifecycle,
+binary message framing for `+SBDWB` / `+SBDI`).
+
+- Iridium developer portal: https://www.iridium.com/services/iridium-sbd/
+- Search the Iridium partner network for the current "9602/9603 Developer
+  Guide" PDF.
+
+Relevant code: `WII5Iridium.cpp`, `WII5SerialManager.cpp` (`+SBDWB`,
+`+SBDI`, `+CSQ` parsing), `WII5BinData.*` (340-byte message format).
+
+## Sparton AHRS-M1 / AHRS-M2
+
+Used as the primary IMU / wave-motion sensor. Key references:
+
+- AHRS-M1 / AHRS-M2 Software Interface Control Document
+- AHRS-M1 / AHRS-M2 Hardware ICD
+- NorthTek Programming Manual (Sparton's Forth-like configuration language;
+  the `programLine()` helpers in `WII5SerialManager.cpp` emit NorthTek
+  commands).
+
+- Sparton (now Bel Power Solutions / part of Bel Fuse) documentation:
+  https://www.belfuse.com/
+
+Relevant code: `WII5Sparton.cpp`, `WII5Setup.cpp` (compass/AHRS init).
+
+## Other components
+
+- **Dallas DS18B20** — temperature sensor:
+  https://www.analog.com/en/products/ds18b20.html
+  (Used in `WII5Weather_18B20.cpp` via the OneWire / DallasTemperature
+  Arduino libraries.)
+- **TinyGPS++** — NMEA parser library:
+  https://github.com/mikalhart/TinyGPSPlus
+- **u-blox NEO-6M / NEO-7M** — GPS module datasheets (NMEA reference):
+  https://www.u-blox.com/
+