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>

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