It is built on a custom engine designed to handle the DS's specific hardware limitations, rather than being a direct port of the GoldSrc engine.
The of the real "Half-Life DS" fan project How homebrew developers ported PC games to the DS
While you cannot find an official cartridge in stores, the "Half-Life DS ROM" remains a staple of the homebrew scene, allowing players to carry the resonance of the Resonance Cascade in their pocket.
A: No. Vicarious Visions’ internal demo has never been leaked. Any website claiming to have it is fraudulent.
Happy headcrab hunting, handheld legend.