Linked zones: shared records kept in step across views

Zones of the same name in different views can be linked. Shared records
are kept in step: a change in one view reaches the others in the same
changeset, as explicit operations, so the diff, permission checks,
history and undo cover every view. Records local to a view override the
shared records of that name and type there; removing the override brings
the shared ones back, and sharing a view's version makes it every view's.
Each zone keeps its own options, serial and file.

Linking existing zones changes no answer: what is the same everywhere
becomes shared, the rest stays local. Unlinking keeps every record, all
local. ACME challenges are written once per link, and --view all changes
a link once. jarl zone link/unlink, zone create --link-with, record
--local/--shared, LINK column, web UI controls, POST /zones/{zone}/link
and /unlink, validation checks linked_*, store migration 11, docs and
e2e.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -97,6 +97,9 @@ error: www.adbraun.lol has a CNAME, so it can't have any other records (found A)
   serves the domain, decided at every renewal. Use them for the web UI
   (HTTPS without a restart) and for named's DNS over TLS and HTTPS (named
   rereads them on renewal), or bring your own.
+* **Linked zones for split DNS**: the same zone in several views, with the
+  shared records kept in step by Jarl and a few records of each view's own
+  (an internal address for a public name, say).
 * **Adopts an existing server**: imports named.conf and zone files
   (comments included), then switches named over with an automatic
   fallback.