Answer 9P on the connection's task, so a session can open its own tree

The editor's loop was the only thing that could answer a 9P request, which
made the editor's own syscalls through a mount of its own tree -- a Look at
/mnt/9p/pardes/<me>/anything under a `9ns --mntgen` view, a Save into it --
requests only the blocked loop could serve. The name-based refusal that
followed (ownMountSuffix) and the in-process routing of a mount of oneself
(Client.sameSession) were patches over that, and both are gone, with the
mailbox that shipped every request to the editor's thread.

One rule replaces them, `pardes.turn`: the core is single-threaded, the
editor's thread has the turn by default and gives it up in two kinds of gap
-- while it waits for input and while a step of it is out in a host syscall
-- and a cloud9 connection task takes it in those gaps to answer. `out`
counts the steps that are out, from any thread: while one is, the core reads
consistently but that step still holds pointers into it, so a request that
would change a pane (a write, a truncation, an rmdir) is parked in the
engine and retried when the turn is next given up with nothing out, and the
editor's own wake waits for the count to reach zero. It is never a write of
its own that a step waits on out there -- writes come from a shell
performing a save between steps -- so a parked request is never the
syscall's own, and making a pane or rendering a screen need not park:
every yield sits before its step's mutation, so the layout and the surface
are whole under it. A changing request that queued effects is answered
once the editor has performed them (`echo Save > exec` returns with the
file written, as acme's `put` does), and it settles the way a step does,
because without that a /log reader waited for the user's next keystroke.

Every host syscall on a user path has to give the turn up, not fs.zig's
alone: the first end-to-end run hung in `inotify_add_watch` performing the
new pane's watch effect. PDFs and images are read whole at open, so no
draw goes out into the host. The core's allocator takes its fixed buffer
through the lock-free interface, since a connection task allocates while
the editor's thread is out in a syscall that allocates too. A Restore puts
the replacement in first and releases every task waiting on the old core.

cloud9 (pinned at eb1a104) parks an open, a truncating wstat, a clunk and a
remove on `again`, not only reads and writes, and answers a parked job
whose fid was clunked without asking the backend.

Verified: test/selfmount.py runs the editor under `9ns --mntgen` and
Looks at, reads and Saves its own tree through the mount; a unit test pins
that a change parks while the editor is out mid-step and lands when it
rests, while a read is answered in the window. 9P over the Unix socket
against a tty session, same machine, Debug builds: a read of /index 278us
-> 61us, a truncating body write 1184us -> 609us, exec Save 718us -> 583us;
the gesture benchmark is unchanged (geometric mean 0.997 over 53 cells).

Also from the reviews: a notice chip over an image or PDF pane was painted
out by the picture drawn after the cells, so pictures give up the rows; in
the GUI a tree-sitter context band painted over the chip, so body layers
are emitted first; a message is one row of printable text, its 256-byte
cut never leaves half a glyph, and one wider than its pane keeps its tail
(the file name, the reason) rather than its head.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

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