Add support for XCX bike (proprietary FFF6 telemetry) with parser, device integration and tests

[cagnulein]

Motivation

• Support XCX bikes which expose FTMS/CSC/Cycling Power but use a proprietary FFF6 telemetry stream that must be handled specially.
• Ensure the XCX-specific detection runs before the generic FTMS detection to avoid misclassification.
• Provide a tested parser for captured FFF6 frames and integrate the device into the test harness and build system.

Description

• Add a new xcxbike device implementation with xcxbike.h and xcxbike.cpp that parses proprietary FFF6 telemetry and exposes metrics via the existing bike interface.
• Implement xcxbike::parseTelemetry to decode the captured 20-byte telemetry frame and update speed, cadence, power, distance, resistance and related state.
• Integrate XCX detection into bluetooth::deviceDiscovered so devices whose name starts with "XCX-" are instantiated as xcxbike before falling back to generic FTMS handling.
• Wire the new device into the project by adding includes and a member pointer in bluetooth.h, update qdomyos-zwift.pri to compile the new sources/headers, and register the device in the test device indices.
• Add unit tests: tst/Devices/TestXcxBikeParser{.h,.cpp} validate decoding of captured packets, and include the test in the test project tst/qdomyos-zwift-tests.pro.

Testing

• Built the project with the new sources and headers (qdomyos-zwift build) which completed successfully.
• Ran the new parser unit tests TestXcxBikeParser which executed the parseTelemetry cases and all assertions passed.
• Ran the test suite including qdomyos-zwift-tests which completed with the updated device index and new tests included (tests passed).

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Codex Task