Give the phone the + it was promised, and the editors behind it

Steps 5 and 6 of docs/adding-hosts-on-the-phone.md, which finishes it. The
phone can now put things in a keychain rather than only read one.

The floating + arrives three designs after it was first asked for. It was
refused twice on the honest grounds that it would open an editor this head had
not got — and that stopped being true when steps 1 to 4 landed. It raises a
sheet rather than a form, because "add" on this screen has been two operations
since groups existed: a host, or a group to file hosts under.

Both editors are cards in the list's own row, swapped for the list rather than
stacked over it, following SnippetsScreen and for the reason written there: a
form on top of the list hides what it is about. There is still no dialog and no
editor screen anywhere on this head. The port and username boxes may be left
empty, and the greyed text inside each is what the host will actually use —
following the group picker as it moves, so the form says what leaving a box
blank will do rather than making it a guess.

The back gesture gains a guard above the switch rather than a case inside it.
An editor is not a screen and has no entry there, and it is strictly nearer:
the sheet sits over the editor's own screen, so back lowers whatever is topmost
before it considers moving between screens.

A group editor needs a way in, and the phone draws no groups panel — so the
heading in the host list gains a pencil. A + that adds groups with no way to
correct one is the same strange thing to ship as one that adds hosts with no
way to correct one, and worse: a group's defaults are what every host beneath
it falls back to.

Scouting step 6 turned up a bug worth more than the prose it was looking for.
ReloadGroupsAsync read the active vault alone, which was a cosmetic limitation
while a group carried only a name — a teammate's host showed under UNGROUPED
and nothing else was lost. Since a group began lending a port, a username and a
binding, the same omission silently dropped all three: that host would dial 22
as nobody while the machine is on 2222 as deploy, with nothing on screen saying
why. The resolution map now spans every readable vault. The editable list stays
narrow, because a row shown across vaults needs a vault id for rename and
delete and two vaults may hold groups with one name — but the map needs none of
that, since it is only ever asked what an id says.

An adversarial review of this change found something I had written a comment
denying. The + was a child of the outer Panel rather than of the list's row, so
it anchored to the bottom of the screen and sat squarely over the new EDIT
button and the lower edge of CONNECT — and because a Panel hit-tests its
last-declared child first, tapping the right end of EDIT would have raised the
add sheet. The comment beside it claimed it was in the list's row precisely so
that could not happen, and manual-checks 8.1 names that exact failure. It is in
Grid.Row 2 now. The same pass caught a connect bar that was disabled rather
than hidden while its own comment said hidden, a scrim that would have flashed
its default pressed chrome across the screen, a fab comment claiming an accent
fill no other button had when Button.primary has it, and a back-gesture remark
naming two view-model properties that do not exist anywhere in the repo.

That last one came from this plan, which invented IsVaultsSurface and
IsConnectionsSurface, and pointed step 6 at a ConnectionsScreen.axaml that has
never existed. Both are corrected in the plan rather than quietly worked
around, along with a fourth "groups are flat" site in README.md that step 1
missed.

The phone's rectangles are not measurable and structurally never will be: the
layout suite is net10.0, the Android head is net10.0-android, and Avalonia's
application is a one-shot process global — so a second head cannot share the
process even if the reference were possible. Everything the sheet and both
editors bind to is shared and is tested headlessly here; the pixels go to
docs/manual-checks.md phase 8, nine checks, which is where this project already
sends what it cannot assert.

Tags remain the one thing asked for that did not ship. Tag is a full item kind
and TagIds merges, encodes and resolves, but no screen draws a chip or offers
to add one, so the tags a client can store are ones nothing here can see. Both
editors carry the set through a save untouched so a client that can set them
does not lose them. Recorded as half-shipped in docs/design-import-gaps.md
rather than quietly left out.

Verified by the whole suite: 1402 tests over nineteen projects, none failing.
Both heads build. Nothing seen on a display — phase 8 is what that costs.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -441,8 +441,18 @@ keychain plus a terminal — and the spike that gates all of it.
   synced items — encrypted, merged and pushed like every other — and both are invisible until used: a
   keychain with no groups draws the flat host list it always did. Two things about them are deliberate.
   Group membership is a field on the *host* rather than a member list on the group, so filing two machines at
-  once on two laptops is two independent writes instead of one contested one; and groups are flat, because a
-  parent pointer merged field by field lets two offline clients build a cycle that nothing can repair.
+  once on two laptops is two independent writes instead of one contested one; and a group carries the port,
+  username and key the hosts inside it fall back to, read at connect time rather than copied in, so changing
+  one changes every host that never overrode it.
+
+  Groups nest, and that reverses an earlier decision worth recording. They were flat because a parent pointer
+  merged field by field lets two offline clients build a cycle nothing upstream can see — the pointer is
+  inside the payload, so the server cannot read it, and the merge resolves one item against one item and
+  never sees the pair. That is still true. What changed is that inheritance made the chain something the
+  *connect* path walks, so the answer had to be a walk that terminates whatever it is handed: every walk
+  carries a visited set and stops at a repeat, which degrades a cycle to a group reading as a root rather
+  than to a shell that never opens. Given a walk that had to be cycle-safe anyway, refusing to nest bought
+  nothing.
 
   Inserting a snippet types it at the prompt and stops. Pressing Enter is a per-snippet decision, off by
   default, and the reason is worth stating: a terminal is one input stream with no notion of being at a