docs: add the status registry implementation plan

Eight tasks against the registry spec: the singleton with the dnd mode,
the statusctl CLI and its check, naming the keepalive window, presentation
mode and its three effects, the module and tile, the page and registration,
installing the CLI and swapping the waybar module, then the README and
traps.

The plan carries the facts probed while designing so the implementer does
not re-probe them: the runtime directory is tmpfs, elogind runs here with
the pam caveat that stops it guaranteeing logout cleanup, the compositor
advertises zwp_idle_inhibit_manager_v1, inotifywait is present, and
Inconsolata Nerd Font lives under ~/.fonts rather than either share
directory. That last one cost a failed cmap search that read as a missing
glyph when it was a bad search path.

Two claims stay marked unverified because they come from documentation and
have not been observed running: that a FileView fires its own fileChanged
on setText, and that IdleInhibitor does nothing without a non-null window.
The plan handles both and tells the implementer to confirm them and record
what actually happens.

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

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