Add isolated FFmpeg movie decoder shim

native/age_movie_ffmpeg/bootstrap-win64.ps1

@@ -0,0 +1,35 @@
+param(
+    [string]$Destination
+)
+
+$ErrorActionPreference = 'Stop'
+$repoRoot = (Resolve-Path (Join-Path $PSScriptRoot '..\..')).Path
+if ([string]::IsNullOrWhiteSpace($Destination)) {
+    $Destination = Join-Path $repoRoot 'build\ffmpeg-sdk'
+}
+$manifest = Get-Content -LiteralPath (Join-Path $PSScriptRoot 'dependency-win64.json') -Raw | ConvertFrom-Json
+$downloadDir = Join-Path $repoRoot 'build\downloads'
+$archivePath = Join-Path $downloadDir $manifest.archive
+$extractRoot = Join-Path $Destination ([IO.Path]::GetFileNameWithoutExtension($manifest.archive))
+New-Item -ItemType Directory -Force -Path $downloadDir | Out-Null
+
+if (-not (Test-Path -LiteralPath $archivePath)) {
+    Invoke-WebRequest -Uri $manifest.url -OutFile $archivePath
+}
+$actualHash = (Get-FileHash -Algorithm SHA256 -LiteralPath $archivePath).Hash.ToLowerInvariant()
+if ($actualHash -ne $manifest.sha256) {
+    throw "FFmpeg archive SHA-256 mismatch: expected $($manifest.sha256), got $actualHash"
+}
+if (-not (Test-Path -LiteralPath $extractRoot)) {
+    Expand-Archive -LiteralPath $archivePath -DestinationPath $Destination
+}
+$sdkRoot = Get-ChildItem -LiteralPath $Destination -Directory |
+    Where-Object { Test-Path -LiteralPath (Join-Path $_.FullName 'include\libavformat\avformat.h') } |
+    Select-Object -First 1 -ExpandProperty FullName
+if (-not $sdkRoot) { throw "FFmpeg SDK was not found under $Destination" }
+$reported = & (Join-Path $sdkRoot 'bin\ffmpeg.exe') -version | Select-Object -First 1
+$expectedVersion = $manifest.ffmpeg_version -replace '-20260721$', ''
+if ($reported -notlike "*$expectedVersion*" ) {
+    throw "Unexpected FFmpeg build: $reported"
+}
+Write-Output $sdkRoot