Add per-user Windows installer and build instructions

installer/Build-Installer.ps1

@@ -0,0 +1,12 @@
+param([string]$IsccPath = "${env:ProgramFiles(x86)}\Inno Setup 7\ISCC.exe")
+$ErrorActionPreference = 'Stop'
+$projectRoot = Split-Path $PSScriptRoot -Parent
+if (!(Test-Path -LiteralPath $IsccPath)) { throw "Inno Setup compiler not found. Supply -IsccPath with the path to ISCC.exe." }
+Push-Location $projectRoot
+try {
+    dotnet publish Orbit.csproj -c Release -r win-x64 --self-contained true -p:PublishSingleFile=false -p:DebugType=None -p:DebugSymbols=false -o release/installer-payload
+    if ($LASTEXITCODE -ne 0) { throw 'Publish failed.' }
+    Copy-Item README.txt release/installer-payload/README.txt
+    & $IsccPath installer/Orbit.iss
+    if ($LASTEXITCODE -ne 0) { throw 'Installer compilation failed.' }
+} finally { Pop-Location }