docs: add bidirectional remote builder setup notes to flake

flake.nix

@@ -117,6 +117,25 @@
 #   Fix: remove from system packages + install via native build
 #   once uConsole is set up as remote builder.
 
+#
+# ==== Remote builder setup (bidirectional) — TODO ====
+# To eliminate cross-compile exceptions, set up distributed builds:
+# 1. Create a dedicated `builder` user on both hosts (no shell, home=/var/empty)
+# 2. Add the same SSH key to both hosts (symmetric)
+# 3. On lazyworkhorse — `nix.buildMachines` pointing to uConsole for aarch64-linux
+# 4. On uConsole — `nix.buildMachines` pointing to lazyworkhorse for x86_64-linux
+# 5. Remove the uconsoleCrossOverlay workarounds above
+# 6. Nix auto-dispatches derivations by `system` — no per-package exceptions needed
+# Example buildMachines config:
+#   nix.buildMachines = [{
+#     hostName = "uConsole.local";
+#     systems = ["aarch64-linux"];
+#     maxJobs = 4;
+#     sshUser = "builder";
+#     sshKey = "/etc/ssh/builder_key";
+#   }];
+#
+
       # ==== How to build natively on uConsole ====
       # To native-compile these on the uConsole:
       # 1. Add uConsole as a remote builder (nix.buildMachines)