feat(waybar): pacdiff + failed-units reminders

Two live waybar modules — no timer/state-file pipeline because the
inputs are cheap to compute on every poll:

custom/pacdiff (interval 300s)
  Counts unresolved .pacnew / .pacsave files via `pacdiff -o` (output
  mode — lists only, takes no action). Hidden at zero. Yellow 'pacdiff
  N' otherwise. Mako fires once on the 0→N transition, so you get
  exactly one nudge per upgrade wave, not a sustained re-nag for files
  you've decided to defer. Click runs `sudo DIFFPROG='nvim -d' pacdiff`
  in a floating ghostty.

custom/failed-units (interval 30s)
  Sums `systemctl --failed` (system) and `systemctl --user --failed`
  counts. Hidden at zero. Red 'failed N' otherwise. Mako fires only on
  upward transition (count went up since last poll), so already-known
  failures don't keep paging you while you investigate. Click prints
  both `systemctl --failed` outputs in a floating ghostty.

Both modules use the same $XDG_RUNTIME_DIR/waybar-X-prev pattern as the
update reminder for state, which makes 'reboot resets the nag' the
default behaviour — exactly the right semantics for both: a fresh boot
deserves a fresh look at pending pacdiffs and any failed units.

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