coordinator: the Magic Trackpad on Bluetooth, off the dock's USB

[mecattaf]

The Magic Trackpad now connects to the coordinator over Bluetooth instead of USB through the dock. It was paired and connected live on 2026-09-17: it binds to the `magicmouse` driver over uhid, and battery shows up in `/sys/class/power_supply`.

• `hosts/coordinator/trackpad.nix` (new):
  • Notes that the bond key lives in `/var/lib/bluetooth` and isn't declared anywhere, so a reinstall loses it.
  • Gives the re-pair steps that worked: trust before pairing, connect right after. The usual scan → pair → trust → connect order left a half bond (`Invalid exchange (52)`, and the trackpad stayed discoverable).
  • Sets `hardware.bluetooth.settings.General.FastConnectable = true` so a click reconnects promptly after the trackpad sleeps. It takes effect after `systemctl restart bluetooth` or a reboot, because a switch doesn't restart bluetooth.service.
• niri: no change. libinput reports `bluetooth:004c:0265` as a touchpad (pointer+gesture), so the existing `touchpad {}` block applies as before. There was never a per-device block.
• `hosts/client/default.nix`: the dock comment no longer lists the trackpad.

Checked: `nixosConfigurations.coordinator` toplevel evaluates and nixfmt passes. Not built or switched yet.

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