CI: Windows build + release on yuuka-windows

tool/package_windows.ps1 builds watch-together-<ver>-windows-x64.zip
(same layout as the Linux tarball, libmpv bundled, update.bat and
watch-together-server.exe). The release workflow gets a windows job;
ci runs the updater tests on Windows. Publishing moves to
tool/release_publish.dart (dart: only, no python/bash on Windows), which
tolerates both jobs racing to create the release and replaces an asset
on re-runs. release.sh writes both asset URLs into the manifest.
Windows jobs check out with plain git (the host's cached
actions/checkout clone was corrupt).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tool/setup_flutter_windows.ps1

@@ -0,0 +1,15 @@
+# CI only (yuuka-windows, host mode): put Flutter stable on PATH for the
+# following steps. The SDK is cached on the host in
+# %ProgramData%\flutter-ci and fast-forwarded each run; first run clones.
+$ErrorActionPreference = 'Stop'
+$F = "$env:ProgramData\flutter-ci"
+if (Test-Path "$F\.git") {
+  git -C $F fetch -q --depth 1 origin stable; if ($LASTEXITCODE) { exit 1 }
+  git -C $F reset -q --hard FETCH_HEAD; if ($LASTEXITCODE) { exit 1 }
+} else {
+  git clone -q https://github.com/flutter/flutter.git -b stable --depth 1 $F; if ($LASTEXITCODE) { exit 1 }
+}
+Add-Content $env:GITHUB_PATH "$F\bin"
+& "$F\bin\flutter.bat" config --no-analytics | Out-Null
+& "$F\bin\flutter.bat" --version
+if ($LASTEXITCODE) { exit 1 }