Show the host keys this vault has approved

Trust was created by the connect prompt and withdrawn from one host's editor, so
a pin for a host that had since been deleted or re-addressed was unreachable
from the interface entirely. It went on refusing connections and nothing in the
application would admit it was there. Two of the four recorded debts were really
this one: leftover pins, and no list to see them in.

A fourth section in the vault column, and the first that adding one has been
cheap for — three edits and two layout tests, which is what #8 and #9 were for.

No editor and no Add, which makes it the only section with neither. A pin is not
something anybody writes: it appears when somebody approves a fingerprint at the
moment of connecting, which is the one place a person can actually check it
against what the operator published. A form for typing one in would be a form
for pasting whatever a man in the middle just offered. So the section exists to
show and to withdraw, which is exactly what was missing.

The fingerprint is shown in full, wrapped, in a monospace line. The only thing
anybody does with one is compare it against a fingerprint an operator published,
and half of one cannot be compared — it can only be glanced at, which is the
habit pinning exists to replace. Nothing here is secret; a host key fingerprint
is published on purpose.

A pin no host in this vault dials is badged rather than hidden or deleted. That
is the leftover the debt was about, and keeping it is still right: the address
may be reached by something without a bookmark, and trust is about the endpoint
rather than the bookmark. The badge is a hint and not a verdict, which is why
nothing acts on it. Matched case-insensitively, because a host name is, and
because a list that called DB.internal unused next to a host saved as
db.internal would be inviting somebody to delete trust they rely on.

Forgetting goes through the same ForgetAsync as the host editor's button, which
withdraws every pin for the address rather than the selected row. Deliberate:
somebody who has stopped trusting a machine has not decided to keep trusting one
of its keys, and a second pin under another algorithm would go on being offered
at the next handshake — which reads as a withdrawal that did not work. The
status line says how many went, and the change is pushed immediately, because
the other machines are the ones still refusing to connect to a rebuilt server.

The list is read through the repository rather than through VaultKnownHostStore,
whose snapshot is shaped for the SSH handshake: one pin per endpoint,
deduplicated, no entity ids. This list has to show duplicates, because a
duplicate is one of the things worth seeing.

Two mutations, both caught: calling every pin dialled (3 tests), and defaulting
the selection to the first row (1) — the same hazard as the credential list,
since Forget acts on the selection.

The selector now holds four buttons in 340 pixels, and TheSelectorIsBigEnoughToClick
measures how much of that they use rather than leaving a fifth section to
discover it as "a button falls outside the window".

936 tests green across 16 projects, 6 of them new. Zero warnings, format clean.

Not verified: how the section looks. It joins the list in outstanding item #7.

tests/DodoSSH.Client.App.Layout.Tests/VaultColumnLayoutTests.cs

@@ -152,6 +152,21 @@ public sealed class VaultColumnLayoutTests : IAsyncLifetime
         await MeasureAsync(faults => faults.ShouldBeEmpty());
     }
 
+    /// <remarks>
+    /// The only section with no editor, so it has only the one shape — but the tallest rows, because each
+    /// carries a full fingerprint on a wrapped monospace line rather than a one-word description.
+    /// </remarks>
+    [Fact]
+    public async Task TheHostKeysSectionFits()
+    {
+        vault.ShowSectionCommand.Execute(VaultSection.KnownHosts);
+        vault.ShowsKnownHosts.ShouldBeTrue();
+
+        vault.KnownHostPins.ShouldNotBeEmpty("an empty list is the easy case and proves nothing here");
+
+        await MeasureAsync(faults => faults.ShouldBeEmpty());
+    }
+
     /// <remarks>
     /// The host editor is the one a third item type made taller: its authentication picker is now a ComboBox
     /// with a two-line-capable item template, and the section it sits in is the only one holding a
@@ -236,6 +251,14 @@ public sealed class VaultColumnLayoutTests : IAsyncLifetime
             column.KeyboardTarget.ShouldBeSameAs(column.CredentialList);
             column.KeyboardTarget.Focus().ShouldBeTrue("the credentials section is showing");
         });
+
+        vault.ShowSectionCommand.Execute(VaultSection.KnownHosts);
+
+        await OnTheColumnAsync((column, _) =>
+        {
+            column.KeyboardTarget.ShouldBeSameAs(column.KnownHostList);
+            column.KeyboardTarget.Focus().ShouldBeTrue("the host keys section is showing");
+        });
     }
 
     /// <remarks>
@@ -266,6 +289,7 @@ public sealed class VaultColumnLayoutTests : IAsyncLifetime
         await AssertOnlyVisibleAsync(VaultSection.Hosts);
         await AssertOnlyVisibleAsync(VaultSection.Keys);
         await AssertOnlyVisibleAsync(VaultSection.Credentials);
+        await AssertOnlyVisibleAsync(VaultSection.KnownHosts);
     }
 
     /// <summary>Shows one section and checks that it is the only one a user can see.</summary>
@@ -280,6 +304,7 @@ public sealed class VaultColumnLayoutTests : IAsyncLifetime
                 [VaultSection.Hosts] = column.HostList,
                 [VaultSection.Keys] = column.KeyList,
                 [VaultSection.Credentials] = column.CredentialList,
+                [VaultSection.KnownHosts] = column.KnownHostList,
             };
 
             foreach (var (owner, list) in lists)
@@ -304,13 +329,24 @@ public sealed class VaultColumnLayoutTests : IAsyncLifetime
         {
             var buttons = column.SectionSelector.Children.OfType<Button>().ToList();
 
-            buttons.Count.ShouldBe(3, "one per section that exists");
+            buttons.Count.ShouldBe(4, "one per section that exists");
 
             foreach (var button in buttons)
             {
                 button.Bounds.Height.ShouldBeGreaterThan(20);
                 button.Bounds.Width.ShouldBeGreaterThan(40);
             }
+
+            // How much room a fifth section would have. The row is a horizontal StackPanel in a 340-pixel
+            // column, so the four labels are close to filling it — and the fit tests above would catch an
+            // overflow only as "a button falls outside the window", which reads as a mysterious layout fault
+            // rather than as "the selector has run out of room". Stated as a number so it reads as itself.
+            var used = buttons.Sum(button => button.Bounds.Width);
+
+            used.ShouldBeLessThan(
+                LayoutHarness.VaultColumnWidth,
+                $"the selector needs {used:0} of {LayoutHarness.VaultColumnWidth:0} pixels; a fifth section "
+                + "means shorter labels or a second row");
         });
     }
 
@@ -375,6 +411,22 @@ public sealed class VaultColumnLayoutTests : IAsyncLifetime
             await vault.SaveCredentialCommand.ExecuteAsync(null);
         }
 
+        // Pins come from approving a fingerprint at connect time, not from an editor, so they are seeded
+        // through the store the connect path writes to. Two for one endpoint, because a host offering keys
+        // of two algorithms is ordinary and the duplicate is one of the things this list has to show.
+        foreach (var (host, algorithm) in new[]
+        {
+            ("host-0.internal", "ssh-ed25519"),
+            ("host-0.internal", "ecdsa-sha2-nistp256"),
+            ("gone.internal", "ssh-ed25519"),
+        })
+        {
+            await knownHosts.TrustAsync(
+                new HostKeyPresentation(
+                    host, 22, algorithm, $"SHA256:{algorithm}-fingerprint-0123456789abcdefghijklmnop"),
+                Token);
+        }
+
         // Back to where the column opens, so every test starts from the state a user would see.
         vault.Section = VaultSection.Hosts;