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/modules/common-nixos.nix

@@ -6,6 +6,13 @@
   time.timeZone = "Europe/London";
   i18n.defaultLocale = "en_GB.UTF-8";
 
+  # Store hygiene. auto-optimise-store hard-links identical files in the store
+  # after each build (cheap dedupe; NOT a garbage collector -- there is
+  # deliberately no automatic GC timer). The larger download buffer avoids
+  # "buffer full" stalls when fetching big NARs over a fast link.
+  nix.settings.auto-optimise-store = true;
+  nix.settings.download-buffer-size = 134217728; # 128 MiB
+
   # Minimal system-level CLI available before the home-manager profile loads
   # (e.g. early boot / rescue). User-level tooling lives in home-manager.
   environment.systemPackages = with pkgs; [