docs: add the status registry design

Desktop modes as state: do not disturb and presentation mode, with one
place that owns them. First of two specs; the notification daemon that
replaces dunst is the second and consumes the registry's dnd mode rather
than owning it. The registry comes first because DND belongs to the
desktop rather than to the notification daemon, and because the daemon is
the larger and riskier component.

The design that needed deciding was not the toggle but the coupling.
Presentation mode sets DND, so DND has two writers, and turning
presentation mode off has to restore what the user had rather than
clearing it. That rule is why this is a registry and not two booleans.

State is one file per mode in XDG_RUNTIME_DIR, matching the convention
breaktimer already writes on this machine. That directory is /run/user/1000,
a tmpfs, so a reboot clears every mode with no cleanup code. Logout without
reboot is less certain and the design does not lean on it: elogind runs
here and its pam module documents removing the runtime directory at last
logout, but the same manual says the module does nothing when the system
was not booted with elogind as init, which on Slackware it is not. If
removal does not happen, a mode survives a logout, which is the same
behaviour as surviving a shell restart.

FileView covers both directions: atomicWrites for the write, watchChanges
for the watch, no shell-out. External watchers need close_write,moved_to
because an atomic write arrives as a rename, the same trap the mail watcher
hit with Xapian.

Confirmed while designing: the compositor advertises
zwp_idle_inhibit_manager_v1 and waybar's built-in idle_inhibitor already
drives it here, so the inhibitor is asserted over Wayland rather than D-Bus.
That module is replaced rather than demoted, because it owns its inhibitor
object and cannot display state owned by anything else; left alongside the
registry it would assert a second inhibitor and idle would resume only when
both released.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01A7ThHHh5iTYbVfp3rNAkw2

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