feat(gb-test-runner): import official MagenTests suite

[pakitovic]

Summary

• Replace the legacy DocBoy-sourced Magen suite manifest with the official magen suite sourced from alloncm/MagenTests 0.5.0 release assets.
• Add file_base_url fetch support for per-file release assets and cover it with manifest/materialization tests.
• Keep the existing local framebuffer fixtures as Magen oracles and leave the magen entry active in test-roms-extra.
• Update ROM-suite references and CGB roadmap notes to remove the old Magen lane.

Validation

• cargo fmt
• cargo rom-fetch magen
• cargo rom-suite magen --suite magen
• cargo rom-report magen
• cargo test -p gb-test-runner
• git diff --check