Refuse a session's own mount, and paint notices as a tagline band

Opening a path inside this editor's own 9P tree hung the session outright, and
it is easy to do by accident: a file manager whose $EDITOR is pardes, or a Look
at anything under /mnt/9p/pardes/<me>/. The realpath, the stat and the read all
leave through the mount and come back as 9P requests only this editor's loop
can answer, while that loop is blocked making them. The filesystem then stops
answering anybody, which is what made it look frozen rather than slow.

The answer has to come from the NAME, before any syscall, because the syscall
is the thing that never returns: the listener notes the name it is posted under
and `resolveOs` refuses a path containing `/pardes/<that name>/`, with
`readLimit` refusing it too for anything that gets past resolution. Another
session's mount stays perfectly usable, and `/n/self/...` is the way to reach
your own tree -- the editor serves it from memory without leaving the process.
Reproduced before and after: the 9P write that never returned now returns, the
editor stays responsive, and it spends four CPU ticks doing it.

The transient lines also now look like what they were modelled on. They carried
the context band's bookkeeping -- one list, rows reserved the way sticky
headers reserve them -- but still painted as ordinary body text, so a message
read as a stray line at the bottom of the pane rather than as part of its
chrome. They take the tagline font and the tagline's own colours now, verified
on a running editor: the announcement lands with font_role=tagline.

Two tests the features never had: a builtin announcing itself, reaching the
notice list, being overridden by Msg's own text and silenced by Verbose; and
the own-mount guard, including that a session with no listener refuses nothing.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

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