fix(remote-builders): serve x86_64 only, drop aquamarine binfmt (#517)

Assisted-by: DeepSeek Harness (deepseek-flash)

hosts/idols-aquamarine/README.md

@@ -269,8 +269,9 @@ group is what requires the recipient step above.
 
 ## Remote builds
 
-Aquamarine is the fleet's remote Nix builder: 24C/96G, plus `boot.binfmt.emulatedSystems` for
-`aarch64-linux` and `riscv64-linux`, so one machine serves all three platforms. Clients opt in with
+Aquamarine is the fleet's remote Nix builder: 24C/96G, serving native `x86_64-linux` only. It runs
+no `boot.binfmt.emulatedSystems`: emulated builds are ~10x slower per core, shoukei builds aarch64
+natively, and this flake has no riscv64 output. Clients opt in with
 `modules.remote-builders.enable`, defined in `modules/nixos/base/remote-builders.nix`.
 
 The builder's login user is `root`, which the daemon trusts by default, so this host needs no

hosts/idols-aquamarine/hardware-configuration.nix

@@ -34,15 +34,6 @@
   # clear /tmp on boot to get a stateless /tmp directory.
   boot.tmp.cleanOnBoot = true;
 
-  # Enable binfmt emulation of aarch64-linux, this is required for cross compilation.
-  boot.binfmt.emulatedSystems = [
-    "aarch64-linux"
-    "riscv64-linux"
-  ];
-  # This enables the kernel to preload the emulator binaries when the binfmt registrations are added,
-  # obviating the need to make the emulator binaries available inside chroots and chroot-like sandboxes.
-  boot.binfmt.preferStaticEmulators = true; # required to work with podman
-
   # supported file systems, so we can mount any removable disks with these filesystems
   boot.supportedFilesystems = [
     "ext4"