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/MacPro31/README.md

@@ -6,10 +6,11 @@ Flake host: `lyrathorpe-macpro31`. Desktop (`portable = false`, imports
 
 ## Hardware configuration
 
-`hardware-configuration.nix` here is a hand-written **placeholder**. On the real
-machine, run `nixos-generate-config`, replace the file, and commit it. It assumes
-by-label partitions — ESP `ESP` (vfat, mounted at `/boot`), root `nixos` (ext4),
-and `swap` — so either label them at install time or swap in the generated UUIDs.
+`hardware-configuration.nix` here is the real config generated by
+`nixos-generate-config` on the machine. Root is an **LVM** logical volume
+(`/dev/mapper/MacPro-Root`, ext4); the ESP (vfat) and swap are referenced by
+UUID. The initrd carries `dm-snapshot` for the LVM root. Regenerate and commit
+if the disk layout changes.
 
 ## Bootloader