Package the Linux desktop head as an RPM, and publish a nightly one

The third desktop platform shipped as nothing: a checkout somebody built.
VelopackUpdateChannel already named the two platforms it serves rather than
negating the one it does not, so that a Linux package would be an addition
there rather than something that had silently half-happened. This is it, and
it is deliberately not a Velopack channel.

There is no updater in the package at all. On this platform dnf is already an
updater the user trusts, audits and controls, and a second one answering to
this project instead is the capability ADR 0013 rule 4 spends a page keeping
unsettable. The cost is real and is written down rather than glossed: nothing
tells a Linux user that a newer build exists, because there is no repository
to tell them.

Rule 3 is untouched. The build job packages the release-shaped RPM on every
main build and every tag and uploads nothing, exactly as it does for Windows
and macOS; the desktop job packages the nightly-shaped one and puts it on the
rolling nightly-desktop release beside the Windows feed. Two RPMs with
different names owning different paths cannot replace one another, so rule 9's
separations come almost free here — the profile split still belongs to the
build, through DodoChannel.

vpk verifies that what it packs actually calls VelopackApp.Build().Run(), and
nothing here does that for us, so scripts/ci-linux-rpm.sh makes its own
assertions: it asks the finished package what it requires, what it provides,
what it holds and what modes those carry. Two of those checks exist because
the first realistic payload tripped them.

  - .NET's libmscordbi.so links libmscordaccore.so. With bundled provides
    excluded — which they must be, or this package would advertise libcoreclr
    to the whole system — that produced a dependency no host on earth can
    satisfy. The requires exclusion is computed from the payload rather than
    listed, because a list of Microsoft's internal file names goes stale in
    the direction that yields an uninstallable package.

  - cp -a inherits the publish directory's modes, and off the wrong filesystem
    that meant a world-writable /opt/DodoSSH: any local account able to
    replace the SSH client every other account runs. For an application
    holding a team's credentials in plaintext that is the trust model walked
    around rather than attacked. Every mode is now set absolutely.

Verified as far as a runner can: both packages install side by side on Fedora
44, resolve webkit2gtk4.1 2.52.5 — the backend MainWindow records as measured
— keep separate launchers, icons and profiles, and dnf remove takes one tree
and leaves the other. 0.0.1~alpha.0.143 sorts below both 0.0.1 and the next
height. What no runner can answer is Phase 19, which is new and unwalked.

RHEL 9 is deliberately not claimed. It ships WebKitGTK's 4.0 line under
libwebkit2gtk-4.0.so.37, and an or-branch for it was written and then removed:
nobody has run the head there, and dnf refusing with "nothing provides
libwebkit2gtk-4.1.so.0" is a better answer than an install that draws a blank
terminal. Same sentence, and the same reason, as the missing Intel Mac.

x86_64 only, for that reason and not a different one.

src/DodoSSH.Client.App/Assets/dodossh-icon.ps1

@@ -225,3 +225,37 @@ $icnsTarget = Join-Path $PSScriptRoot 'dodossh.icns'
 $icns.Dispose()
 
 Write-Output "Wrote $icnsTarget ($($icnsTypes.Count) entries, $((Get-Item $icnsTarget).Length) bytes)"
+
+# ---- dodossh-<size>.png, for the Linux desktop entry ---------------------------------------------
+#
+# The third platform, and the only one that wants the raster loose rather than in a container. A
+# freedesktop icon theme is a directory tree — hicolor/48x48/apps/dodossh.png and friends — and the
+# RPM built by scripts/ci-linux-rpm.sh installs one file per size into it.
+#
+# Written here for the reason this whole file exists: one geometry, regenerated in one command. The
+# alternative was to have the packaging script pull the 256px frame out of dodossh.ico at build time,
+# which works — the .ico's entries really are PNGs — and puts a second place that has to know how an
+# ICO directory is laid out. It also silently ties the Linux icon to whatever sizes Windows happens
+# to want.
+#
+# ◆ THE TILE BLEEDS TO THE EDGE HERE, AS IT DOES ON WINDOWS.
+#
+# tileFraction stays 1.0. The 0.8047 inset above is Apple's icon grid and nothing else — GNOME, KDE
+# and every panel that reads hicolor draw the icon at the size they asked for, with no shadow and no
+# magnification to leave air for. An inset icon would simply be the small one in the launcher.
+#
+# ◆ AND THE SIZES ARE NOT A ROUND NUMBER OF THEM.
+#
+# 48 is the size a GNOME dash and most panels ask for, 128 and 256 are what a large-icon file manager
+# and GNOME Software use, and 64 is between them because the 48 upscaled to it is visibly soft. There
+# is deliberately nothing below 48: the hairline floor in New-MarkPng keeps a 16px mark legible, but
+# no Linux desktop asks this theme for one — window titlebars there draw no icon at all.
+$linuxSizes = @(48, 64, 128, 256)
+
+foreach ($size in $linuxSizes)
+{
+    [byte[]]$png = New-MarkPng $size
+    $pngTarget = Join-Path $PSScriptRoot "dodossh-$size.png"
+    [System.IO.File]::WriteAllBytes($pngTarget, $png)
+    Write-Output "Wrote $pngTarget ($((Get-Item $pngTarget).Length) bytes)"
+}