feat(desktop): address a request to a screen, instead of asking the room

[Nanguage]

Directed requests — desktop_read / desktop_call / desktop_screenshot and
everything else through _desktop_request — now resolve the chat's anchor
(#192) and stamp the event with that viewport's id. Every other viewport hears
it and ignores it.

That retires the Web Locks election for these. The election worked, but
anonymously: a copy acted and nobody could say afterwards which one had,
which was the single hardest thing about debugging this layer.

Two limits fall away with it.

The ping is gone. It existed because a fresh chat's first request could beat
the UI's subscription to that chat's stream, and streams do not replay — so
every directed request paid a handshake round trip first. The stream is
pod-scoped now, and a lease is better evidence of a live screen than a probe
that races. _desktop_ping and _publish are deleted with their last callers.

A chat context is no longer required. _desktop_request used to refuse with
"no chat context — cannot reach the desktop", which is why desktop_open could
not be called from a standalone desktop page at all. It now resolves to the
pod's most recently active viewport.

The failure text is specific for the same reason: "no desktop to ask — no
viewport is open on this pod" sends someone to open one, where "the desktop
did not answer" sent them looking for a bug in a desktop that was simply not
open.

Verified on staging (sha-97b3648):

desktop_open from a page with no chat	ok, win-30 — previously a hard refusal
desktop_anchor vs the page's real __pantheonViewportId	identical
desktop_read on a Terminal window	correctly refuses: not a packaged app, so no bridge — anchoring did its job
desktop_windows with no desktop open at all	answered from the document

71 desktop tests pass. Browser half is on pantheon-ui feat/absorb-atrium: it
acts when addressed, and falls back to the old election only when an event
carries no viewport_id (a pod that predates this).