Prioritize XDev hand tracker sources

[nv-jakob]

Closes #344

This is a temporary fix until the CloudXR runtime has the right behaviour.

Summary by CodeRabbit

Release Notes

• New Features
  • Enhanced hand tracking to support additional device types, providing improved compatibility and more robust fallback mechanisms.

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📝 Walkthrough

Walkthrough

This PR extends LiveHandTrackerImpl to support optional OpenXR MNDX xdev device candidates for hand tracking. The implementation enumerates available xdev devices, classifies them as display or preferred devices via property string heuristics, and attempts to create hand trackers scoped to each xdev. If xdev trackers cannot be created, the code falls back to default hand trackers. Runtime update logic iterates over tracker candidates per side, returning on the first active tracker, with proper resource cleanup in both success and failure paths. The build configuration links against OpenXR's loader to enable xdev APIs.

Estimated code review effort

🎯 4 (Complex) | ⏱️ ~45 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 5.56% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'Prioritize XDev hand tracker sources' accurately reflects the main change: adding XDev device support as a priority hand tracker source in LiveHandTrackerImpl.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@src/core/live_trackers/cpp/live_hand_tracker_impl.cpp`:
- Around line 316-323: Before calling pfn_create_hand_tracker_ and push_back,
ensure the vector cannot reallocate: call trackers.reserve(trackers.size() + 1)
prior to creating the XrHandTrackerEXT so that if push_back would otherwise
trigger reallocation it won't and the newly-created handle won't be leaked;
apply the same reserve-before-create change for the other identical creation
site (the second XrHandTrackerEXT creation block) that uses session,
create_info, pfn_create_hand_tracker_, and trackers.

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📥 Commits

Reviewing files that changed from the base of the PR and between d06147d and 97be5f0.

📒 Files selected for processing (3)

• src/core/live_trackers/cpp/CMakeLists.txt
• src/core/live_trackers/cpp/live_hand_tracker_impl.cpp
• src/core/live_trackers/cpp/live_hand_tracker_impl.hpp