feat(nixos): disk hygiene, dedupe shared options, fix MacPro docs

- common-nixos: nix.settings.auto-optimise-store + larger download buffer.
- workstation: fstrim, boot.tmp.cleanOnBoot, and the shared graphical
  options moved here from the per-host configs (pipewire, swaylock PAM
  stub, redistributable firmware) -- MBP-Asahi gains audio it lacked.
- T400: zramSwap for the low-RAM host.
- MBP-Asahi: nixos-apple-silicon binary cache substituter.
- MacPro31 README: describe the real (LVM/UUID) hardware config; it is no
  longer a placeholder.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

system/machine/MBP-Asahi/configuration.nix

@@ -14,9 +14,18 @@
 
   networking.hostName = "Emma-Asahi";
 
-  # No fingerprint reader on this machine; empty service still lets swaylock
-  # authenticate via password.
-  security.pam.services.swaylock = { };
+  # Audio (PipeWire) and the swaylock PAM stack are inherited from
+  # workstation.nix. hardware.enableRedistributableFirmware is also set there;
+  # it is harmless here since Asahi supplies its own peripheral firmware below.
+
+  # Binary cache for the Asahi kernel/build artifacts, so the MBP pulls prebuilt
+  # outputs instead of compiling the Asahi kernel locally.
+  nix.settings = {
+    substituters = [ "https://nixos-apple-silicon.cachix.org" ];
+    trusted-public-keys = [
+      "nixos-apple-silicon.cachix.org-1:8psDu5SA5dAD7qA0zMy5UT292TxeEPzIz8VVEr2Js20="
+    ];
+  };
 
   # Apple peripheral firmware (Wi-Fi/Bluetooth). The directory is gitignored and
   # populated out-of-band -- see README.