feat(packaging): an MSIX target for Microsoft Store submission

Store certification of the NSIS "MSI/EXE" submission failed on three
checks (silent-install verification, Add/Remove Programs entry, bundleware
check) -- traced and reproduced live to one cause: SmartScreen blocks an
unsigned, internet-downloaded installer at the shell layer before
Microsoft's own unattended validation bot ever gets to run it. MSIX
sidesteps this class of failure entirely: submitted through the Store's
native pipeline, there is no browser-download-then-launch step for
SmartScreen to intercept, and Microsoft signs the package itself at
publish time -- free, and specific to this submission type (Trusted
Signing remains a paid service for the MSI/EXE path). Full plan and
findings: C:\Users\admin\devel\msix-installer.md (out of repo).

electron-builder.msix.yml carries the same bundle as Full (node runtime,
ffmpeg, both service scripts) -- an AppX/MSIX install never elevates, by
design, but that changes only *when* the two elevated operations can run,
not whether the daemon ships. No main.js changes were needed: the on-demand
elevation path for service-mode (winElevateServiceMode(), driven from the
Node page) already existed for a different reason and depends only on
service-mode.ps1 being present as an extraResource, true for any packaged
Windows target. identityName/publisher/publisherDisplayName are the real
values from Partner Center's app-identity reservation, not placeholders.

build-win-msix.ps1 points electron-builder at the system Windows 10 SDK
(auto-detected) instead of letting it download its own bundled copy --
that download's 7z extraction creates symlinks this target never uses and
fails without SeCreateSymbolicLinkPrivilege, reproduced on this machine.
build/appx/ carries the four tile images the AppX target requires
regardless of showNameOnTiles, generated once from the existing app icon
(see that directory's README) since the system-SDK redirect has no vendor
samples to fall back to. build/appx-extensions.xml declares
windows.startupTask by hand rather than via electron-builder's
addAutoLaunchExtension, which always targets the Electron shell -- this
points at the bundled node binary instead, matching what "starts at sign
in" already means for Full.

Verified live via a signed sideload install (self-signed test cert,
cleaned up after): the package installs and the app runs correctly. One
finding worth carrying forward -- the declared network capabilities
(internetClientServer, privateNetworkClientServer) do not create any
firewall exemption for this app, most likely because automatic
capability-based exemption is an AppContainer-sandbox property and this
app deliberately runs full-trust, outside any sandbox. Not a regression:
no install-time elevation was possible either way, so the cost is the same
one-time OS firewall prompt firewall.ps1's own header already documents as
its fallback today.

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

7 files changed, 50 insertions, 2 deletions

diff --git a/packages/meshbay-client/build/appx-extensions.xml b/packages/meshbay-client/build/appx-extensions.xml
new file mode 100644
index 0000000..1ee1cca
--- /dev/null
+++ b/packages/meshbay-client/build/appx-extensions.xml
@@ -0,0 +1,3 @@
+  <desktop:Extension Category="windows.startupTask" Executable="app\resources\node-runtime\meshbay-node.exe" EntryPoint="Windows.FullTrustApplication">
+    <desktop:StartupTask TaskId="MeshBayNodeStartup" Enabled="true" DisplayName="MeshBay Node" />
+  </desktop:Extension>
diff --git a/packages/meshbay-client/build/appx/README.md b/packages/meshbay-client/build/appx/README.md
new file mode 100644
index 0000000..ef47411
--- /dev/null
+++ b/packages/meshbay-client/build/appx/README.md
@@ -0,0 +1,44 @@
+# AppX/MSIX tile images
+
+Generated once from `../icon-square.png` (1024×1024), not by any build script
+— electron-builder's AppX target (`app-builder-lib/out/targets/AppxTarget.js`)
+requires these four regardless of `showNameOnTiles`, and looks for them here
+(`packager.getResource(undefined, "appx")` resolves to `build/appx/`) before
+falling back to its own vendor-bundled samples. Falling back was not an
+option here: this repo points `ELECTRON_BUILDER_WINDOWS_KITS_PATH` at the
+system Windows SDK instead of letting electron-builder download its own
+copy (see `packaging/win/build-win-msix.ps1`'s comment for why), and the
+system SDK has no `appxAssets` samples to fall back to at all.
+
+| File | Size | Source |
+|---|---|---|
+| `StoreLogo.png` | 50×50 | resized |
+| `Square44x44Logo.png` | 44×44 | resized |
+| `Square150x150Logo.png` | 150×150 | resized |
+| `Wide310x150Logo.png` | 310×150 | icon centered on a `#464646` canvas (the appx target's own default `backgroundColor`) |
+
+Regenerate with System.Drawing if the source icon changes:
+
+```powershell
+Add-Type -AssemblyName System.Drawing
+$img = [System.Drawing.Image]::FromFile("build\icon-square.png")
+function Resize-Square($image, $size, $outPath) {
+    $bmp = New-Object System.Drawing.Bitmap $size, $size
+    $g = [System.Drawing.Graphics]::FromImage($bmp)
+    $g.InterpolationMode = [System.Drawing.Drawing2D.InterpolationMode]::HighQualityBicubic
+    $g.DrawImage($image, 0, 0, $size, $size)
+    $bmp.Save($outPath, [System.Drawing.Imaging.ImageFormat]::Png)
+    $g.Dispose(); $bmp.Dispose()
+}
+Resize-Square $img 50  "build\appx\StoreLogo.png"
+Resize-Square $img 44  "build\appx\Square44x44Logo.png"
+Resize-Square $img 150 "build\appx\Square150x150Logo.png"
+
+$bmp = New-Object System.Drawing.Bitmap 310, 150
+$g = [System.Drawing.Graphics]::FromImage($bmp)
+$g.InterpolationMode = [System.Drawing.Drawing2D.InterpolationMode]::HighQualityBicubic
+$g.Clear([System.Drawing.Color]::FromArgb(255, 0x46, 0x46, 0x46))
+$g.DrawImage($img, 90, 10, 130, 130)
+$bmp.Save("build\appx\Wide310x150Logo.png", [System.Drawing.Imaging.ImageFormat]::Png)
+$g.Dispose(); $bmp.Dispose(); $img.Dispose()
+```
diff --git a/packages/meshbay-client/build/appx/Square150x150Logo.png b/packages/meshbay-client/build/appx/Square150x150Logo.png
new file mode 100644
index 0000000..cd64829
--- /dev/null
+++ b/packages/meshbay-client/build/appx/Square150x150Logo.png
diff --git a/packages/meshbay-client/build/appx/Square44x44Logo.png b/packages/meshbay-client/build/appx/Square44x44Logo.png
new file mode 100644
index 0000000..d714c94
--- /dev/null
+++ b/packages/meshbay-client/build/appx/Square44x44Logo.png
diff --git a/packages/meshbay-client/build/appx/StoreLogo.png b/packages/meshbay-client/build/appx/StoreLogo.png
new file mode 100644
index 0000000..2062b36
--- /dev/null
+++ b/packages/meshbay-client/build/appx/StoreLogo.png
diff --git a/packages/meshbay-client/build/appx/Wide310x150Logo.png b/packages/meshbay-client/build/appx/Wide310x150Logo.png
new file mode 100644
index 0000000..0ca324d
--- /dev/null
+++ b/packages/meshbay-client/build/appx/Wide310x150Logo.png
diff --git a/packages/meshbay-client/package.json b/packages/meshbay-client/package.json
index 614e6d6..6d8e711 100644
--- a/packages/meshbay-client/package.json
+++ b/packages/meshbay-client/package.json
@@ -1,7 +1,7 @@
 {
   "name": "meshbay-client",
   "version": "0.13.0",
-  "description": "MeshBay desktop client \u2014 the interface ships with the application, not from the hub",
+  "description": "MeshBay desktop client — the interface ships with the application, not from the hub",
   "license": "AGPL-3.0-or-later",
   "author": "MeshBay Team <team@meshbay.org>",
   "homepage": "https://meshbay.org",
@@ -12,7 +12,8 @@
     "sync-ui": "node scripts/sync-ui.js",
     "dist": "bash ../../packaging/build/build-client.sh",
     "dist:win": "powershell -NoProfile -ExecutionPolicy Bypass -File ../../packaging/win/build-win.ps1",
-    "dist:win:light": "powershell -NoProfile -ExecutionPolicy Bypass -File ../../packaging/win/build-win-light.ps1"
+    "dist:win:light": "powershell -NoProfile -ExecutionPolicy Bypass -File ../../packaging/win/build-win-light.ps1",
+    "dist:win:msix": "powershell -NoProfile -ExecutionPolicy Bypass -File ../../packaging/win/build-win-msix.ps1"
   },
   "devDependencies": {
     "electron": "^44.2.0",