Advertise the editor in the posted-9P registry

pardes spoke 9P and could be mounted, but only by naming its socket:
$XDG_RUNTIME_DIR/pardes-9p-<name>.sock sits one directory above the
registry and nothing could find it. Now a listening editor advertises
itself at $XDG_RUNTIME_DIR/9p/pardes/<name>, a symlink to the socket it
already binds. One directory for the program, one entry per editor —
the layout zmx posts its sessions under — so several editors group
rather than crowd the registry root.

The socket does not move: adopting the registry only advertises. Only
the runtime-directory socket posts, so an instance on the
~/.local/state fallback stays out of the user's registry, the way a
private ZMX_DIR does for zmx. Stopping unposts, and only while the
entry is still ours, so a name another editor has since claimed is
never unlinked. The cloud9 pin moves to 9c4d668c for cloud9.post's
path helpers.

Serving is the whole of it. Consuming the registry is 9ns's job: it
mounts the lot at /mnt/9p and an interactive fish already self-wraps in
one, so a pardes started from a terminal reads /mnt/9p/harness/... with
the same code that reads any other path. Two drafts that taught
`resolve` to dial the registry itself were reverted — one duplicated
9ns for no gain, the other reinterpreted relative dials, which are a
feature. `resolve` is byte-identical to what it was, and no dial that
worked changes meaning.

What pardes still does not do, and why, is in docs/cloud9.md: it binds
its own socket rather than posting through cloud9.post, because post
claims flat names only — legalName rejects '/', and claimName derives
its lock directory by stripping "/9p" — so a name inside a subdirectory
cannot go through it. zmx hand-rolls the same symlink for the same
reason. Unifying them means teaching post a group, which is a change to
adversarially-hardened code rather than a rename.

docs/divergences.md records what this bookmark move leaves beside it:
the editor line rruwvuzm (~1300 lines, forked at 01104e7c, still on the
old cloud9 pin), the other bookmarks, and two failures that are not
this change — fs-test's syntax-highlighting assertion, which fails
identically on a clean main, and pardes not starting headless, which is
why this is covered by 9p-io-test rather than by running the editor.

Tests: 11/11 9p-io-test, including a listener that posts on start and
unposts on stop; 31/31 unit-test.

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