Core already owns a chat text size preference ($chatTextScale, Appearance ->
Chat Text Size, presets 90..175). Add it to the host.settings allowlist with
read, write and subscribe, bound to the same store atom and setter the native
control uses, so a theme/plugin like Hermes Quiet can follow or set reading
size without owning layout. Writes accept only the exact presets: the store
would otherwise snap a typo to the default and silently reset the user's size.
Docs: document the key, and list chat width / turn spacing / row geometry as
not-yet keys (no core preference exists; chat width tracks #55287).
Refs #131943#131944#131945
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