Refresh stale architecture docs

CLAUDE.md's "Composition pattern" section still described the
pre-refactor layout (hosts/<host>/configuration.nix as a thin imports
list, hardware-configuration wired in from flake.nix) from before the
platform x build-type matrix landed. Rewrite it to match the current
mkTarget/host.nix architecture and the module moves from the prior
commit. Also fixes docs/nix-cache.md, which referenced a
modules/nix/ path that never existed in this repo.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01La55Nsss8jZ7ZuzUV9mfot

docs/nix-cache.md

@@ -8,8 +8,8 @@ This repository configures `nix-cache` as a **binary cache server** and a **remo
 - Every machine still keeps and uses its own local `/nix/store`.
 - Clients prefer `http://nix-cache` for substitutes and keep `https://cache.nixos.org/` as fallback.
 - Clients can offload builds to `nix-cache` through SSH (`nix.distributedBuilds`).
-- Client hosts import `modules/nix/cache-client.nix` and, when remote building is enabled, `modules/nix/remote-builder-client.nix`.
-- The `nix-cache` host imports `modules/nix/cache-server.nix`.
+- Client hosts import `modules/nix-cache/client.nix` and, when remote building is enabled, `modules/nix-cache/remote-builder-client.nix`.
+- The `nix-cache` host imports `modules/nix-cache/server.nix`.
 
 ## Binary cache signing keys (on nix-cache)