docs(nix): clarify file placement terms

docs/nix.md

@@ -48,6 +48,25 @@ Use:
 dotfiles.mkHost { hostName = "neogaia"; }
 ```
 
+## File Placement
+
+### Put files in the directory that owns their role
+
+Choose the file location before writing a Nix file.
+Put assembly files under `flake/`.
+Do not add library files until this repository has a library directory.
+Put modules under `modules/`.
+Put host configurations under `hosts/<name>/`.
+Put host-side guest wiring under `guests/`.
+Put registry definitions under `registry/`.
+Keep `flake.nix` focused on inputs and public output wiring.
+Do not add root-level Nix files other than `flake.nix`.
+
+A module is self-contained configuration under `modules/`.
+It usually scopes to one program, service, or subsystem.
+A Nix module is any Nix language unit that declares or sets module-system options.
+Files under `hosts/`, `guests/`, and `modules/` return Nix modules.
+
 ## Function Arguments
 
 ### Use closed interfaces for assembly files and library files
@@ -177,40 +196,7 @@ Use:
 lib.mapAttrs f attrs
 ```
 
-## Flake Assembly
-
-### Keep assembly code under `flake/`
-
-Keep flake assembly code under `flake/`.
-Keep `flake.nix` focused on inputs and public outputs.
-The `outputs` function returns each public output as an attribute.
-An external flake reads public outputs through `inputs.<name>.<output>`.
-`nix flake show` displays public outputs.
-
-Avoid:
-
-```nix
-# flake.nix
-let
-  mkHosts = hostsDir: ...;
-in
-{
-  nixosConfigurations = mkHosts (self + "/hosts");
-}
-```
-
-Use:
-
-```nix
-# flake.nix
-let
-  inherit (nixpkgs) lib;
-  flake = import ./flake { inherit lib inputs self; };
-in
-{
-  nixosConfigurations = flake.mkHosts (self + "/hosts");
-}
-```
+## Flake Outputs
 
 ### Expose only supported flake outputs