Tracking: headless VR keyboard injection + diagnostics (imgui-vr-helper)

Tracking issue — no devbench-core changes needed for this one.

**Implementation issue**: alandtse/imgui-vr-helper#47 — headless keyboard injection +
centralized client keyboard diagnostics, so VR-menu input/keyboard bugs (currently requiring
a human in a headset per iteration) can be reproduced and bisected via devbench alone.

Both proposed PRs stay entirely within imgui-vr-helper's existing
`RegisterTool`/`RegisterToolExtension` surface against devbench's current API — devbench's own
`ToolRegistry`/adapters need no changes. Filed here too per request, purely for cross-repo
discoverability.