pkg: package identity so Dynamic Lighting grants background control

The Settings "Background light control" list only offers apps that have
BOTH package identity and the com.microsoft.windows.lighting
windows.appExtension in their manifest (declaration verified against
Lenovo's LegionLightingController manifest) — without a grant, Windows
hands the LEDs to vRIO only while it is the foreground window, i.e.
never during gameplay.

pkg/AppxManifest.xml is a sparse package (external location) that
grants the existing VRio.App.exe identity as-is: win32App runtime
behavior, runFullTrust + unvirtualizedResources, and the lighting
app-extension. Register-vRIO.ps1 registers/unregisters it against any
exe folder (repo build or an extracted dist zip); unsigned registration
needs Developer Mode. Identity is granted through shell activation
only — launch from the Start menu entry or an AUMID taskbar pin, not by
double-clicking the exe (the script prints this warning).

Verified end-to-end on this machine: process reports the package full
name, vRIO appears in the background-control list, and the wire log
shows the withheld → available transition once granted.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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