Handle missing private assets in iOS Actions workflow

[Copilot]

The iOS Actions job failed during asset preparation because the checked-out private repository did not contain default.xex. The workflow now handles unavailable private assets without failing the job.

• Asset preparation
  • Records whether required game assets are available.
  • Emits a warning and exits cleanly when default.xex is absent.
• Conditional build
  • Gates Xcode setup, compilation, IPA packaging, and artifact upload on asset availability.
  • Retains the existing build path when assets are present.

if: steps.prepare-assets.outputs.available == 'true'