v0.20.1: static Linux binaries, steady mirror cursor

Fully static Linux binaries. The released Linux binaries (and the Docker image)
are now statically linked — no libc dependency, so one binary runs anywhere:
Alpine, distroless, or a host with an older glibc. Especially handy for the push
client — scp it onto any box and run it. macOS binaries are unchanged.

Steady cursor in the web mirror. Apps that bracket each redraw in hide/show
cursor sequences — a "thinking" spinner does this ~14×/s — made the mirrored
cursor flicker on and off, because the 30fps sampling caught the brief hidden
gap that the local terminal never shows. The mirror now holds the cursor
steady through a redraw, matching the terminal. Fixed in the producer, so it
takes effect once the serve/push side is on v0.20.1 (no viewer change).

Window title as an event in embedded mode. The iframe-less (light/shadow)
embeds never touch the host page's tab title; the session's window title now
rides a shellglass-title CustomEvent on the element
(event.detail = the title, "" on clear), so the host page decides what to do
with it:

el.addEventListener("shellglass-title", e => { document.title = e.detail })

No protocol version bump — clients, hubs and viewers interoperate unchanged.
Hub operators: the title event ships in the viewer.js / embed.js the hub
serves, so deploying this delivers it to embedded sessions; the steady-cursor
fix lives in the pusher binary instead, so a pusher gets it by upgrading to
v0.20.1.