wip(rpi4): park the Pi 4 sketch — superseded by the Pi 5 machinery in #87

Never-committed working tree state found in the dev worktree: a Pi 4 hardware
module, a setup walkthrough, an upboard-serial refactor, and flake wiring that
does not evaluate (aarch64 packages block nested inside packages.x86_64-linux;
pkgs-aarch64 built with `inherit aarch64` instead of `system`, so it is really
x86; references a nixosConfigurations.rpi4-installed that is never defined).

Parked verbatim for reference. The real Pi 4 target is rebuilt on top of #87's
mkPiInstalled/mkPiImage shape in feat/rpi4-target.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013A6683cCHnQxFUosx1krY4

flake.lock

@@ -435,6 +435,22 @@
         "type": "github"
       }
     },
+    "nixpkgs-aarch64": {
+      "locked": {
+        "lastModified": 1782847189,
+        "narHash": "sha256-twXPFqFsrrY5r28Zh7Homgcp2gUMBgQ6WDS98Q/3xFI=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "b6018f87da91d19d0ab4cf979885689b469cdd41",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-25.11",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
     "nixpkgs-regression": {
       "locked": {
         "lastModified": 1643052045,
@@ -504,6 +520,7 @@
         "devenv": "devenv",
         "flake-utils": "flake-utils",
         "nixpkgs": "nixpkgs_3",
+        "nixpkgs-aarch64": "nixpkgs-aarch64",
         "nixpkgs-unstable": "nixpkgs-unstable",
         "rust-overlay": "rust-overlay"
       }