By: Ammar <ammar.nanjiani@gmail.com>
docs(adr): record the Steam Machine Jovian fork pin decision ADR-015 covers replacing stock SteamOS with Jovian NixOS while PR #584 is still unmerged, and why the pin is isolated as its own flake input so the Steam Deck keeps chaotic's released Jovian. Records the honest costs: an unreviewed branch that can move or be deleted (which would break flake evaluation fleet-wide), "initial" only meaning kernel plus firmware plus the AMD GPU flag, no HDMI-CEC because the branch that restores it also drops firmware signature verification, and default-on fan control and Valve Mesa forks inherited from the Deck. Supersede this once PR #584 merges.