ci: faster SDK download, supersede stale runs

Run 9's Invoke-WebRequest download of the ~1GB SDK was very slow. Use
curl.exe (bundled on Windows 10+) which streams to disk, with an IWR
fallback. Add a concurrency group so a new push cancels an in-progress
run instead of queuing behind it.

.gitea/workflows/ci.yml

@@ -8,6 +8,12 @@ on:
     branches: [main]
   workflow_dispatch:
 
+# Supersede an in-progress run when a new commit is pushed (so a slow run does
+# not block the next iteration). Ignored by older Gitea, harmless if so.
+concurrency:
+  group: windows-ci
+  cancel-in-progress: true
+
 # Targets a self-hosted Windows runner inside mainland China. See README
 # "Continuous integration" for runner prerequisites. Key points:
 #   * github.com is unreachable here, so the checkout action is pulled from the
@@ -66,9 +72,19 @@ jobs:
             New-Item -ItemType Directory -Force -Path $root | Out-Null
             Write-Host "Downloading Flutter SDK from $url"
             $ok = $false
-            for ($i = 1; $i -le 3 -and -not $ok; $i++) {
-              try { Invoke-WebRequest -Uri $url -OutFile $zip -UseBasicParsing; $ok = $true }
-              catch { Write-Host "download attempt $i failed: $_"; Start-Sleep 5 }
+            # curl.exe (bundled on Windows 10+) streams to disk and is far
+            # faster than Invoke-WebRequest for a ~1GB file. Fall back to IWR.
+            $curl = "$env:SystemRoot\System32\curl.exe"
+            if (Test-Path $curl) {
+              & $curl -L --fail --retry 3 --retry-delay 5 -o $zip $url
+              if ($LASTEXITCODE -eq 0 -and (Test-Path $zip)) { $ok = $true }
+              else { Write-Host "curl download failed (exit $LASTEXITCODE), falling back" }
+            }
+            if (-not $ok) {
+              for ($i = 1; $i -le 3 -and -not $ok; $i++) {
+                try { Invoke-WebRequest -Uri $url -OutFile $zip -UseBasicParsing; $ok = $true }
+                catch { Write-Host "IWR attempt $i failed: $_"; Start-Sleep 5 }
+              }
             }
             if (-not $ok) { Write-Error "Failed to download Flutter SDK"; exit 1 }
             Write-Host "Extracting to $root (this is slow on first run)"