Add XCX bike driver (proprietary FFF6 telemetry) and parser tests

[cagnulein]

Motivation

• Support XCX bikes which expose FTMS/Cycling Power/CSC but use a proprietary FFF6 telemetry stream that must be handled specially and not treated as generic FTMS.
• Add a dedicated driver so telemetry, cadence/crank events and device-specific resistance/power interpretation are correct.

Description

• Add a new driver implementation devices/xcxbike/xcxbike.h and devices/xcxbike/xcxbike.cpp that parses the proprietary FFF6 telemetry, manages BLE service/characteristic discovery, enables notifications and publishes metrics and virtual device support.
• Integrate the driver into device discovery by adding a name-specific branch in src/devices/bluetooth.cpp that matches XCX- and instantiates xcxbike, and remove XCX- from the earlier generic FTMS matching list.
• Expose the new device type in headers and build by adding #include "devices/xcxbike/xcxbike.h" to bluetooth.h and adding devices/xcxbike/xcxbike.cpp / .h to qdomyos-zwift.pri and test project files.
• Add unit tests tst/Devices/TestXcxBikeParser.* to validate xcxbike::parseTelemetry against captured packets and rejection cases, and update test device indexes to include the XcxBike device.

Testing

• Added XcxBikeParserTest which exercises xcxbike::parseTelemetry with multiple captured FFF6 packets and XcxBikeParserRejectionTest for malformed inputs, and these tests pass locally.
• Ran the test suite including the new parser tests via the project's test runner (ctest/test target) and observed all tests succeeding.

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Codex Task