chore(windows): persistent build env + one-call host/web deploy scripts

scripts/windows/: setup-build-env.ps1 persists the NVENC build env (Machine scope:
PUNKTFUNK_NVENC_LIB_DIR, LIBCLANG_PATH, CMAKE_POLICY_VERSION_MINIMUM -- no FFMPEG_DIR, the
nvenc build doesn't link libavcodec). deploy-host.ps1 rebuilds --release --features nvenc and
restarts the PunktfunkHost service with .bak rollback on build/start failure. build-web.ps1
rebuilds the Nitro web console (bun build, node runtime) and restarts the PunktfunkWeb task.
README documents the flow -- a redeploy is now a single script call.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/windows/setup-build-env.ps1

@@ -0,0 +1,27 @@
+<#
+  Persist the punktfunk Windows-host build environment (Machine scope, run once, elevated).
+
+  After this, deploy-host.ps1 needs only the VS toolchain (it loads vcvars64.bat itself).
+  These are BUILD-time vars; runtime config lives in C:\ProgramData\punktfunk\host.env.
+
+    powershell -ExecutionPolicy Bypass -File scripts\windows\setup-build-env.ps1
+#>
+$ErrorActionPreference = 'Stop'
+
+$admin = ([Security.Principal.WindowsPrincipal][Security.Principal.WindowsIdentity]::GetCurrent()
+         ).IsInRole([Security.Principal.WindowsBuiltinRole]::Administrator)
+if (-not $admin) { throw "Run elevated (Machine-scope env requires Administrator)." }
+
+# NVENC import lib (nvencodeapi.lib); libclang for bindgen; cmake policy floor for audiopus_sys.
+$vars = [ordered]@{
+  'PUNKTFUNK_NVENC_LIB_DIR'      = 'C:\Users\Public\nvenc'
+  'LIBCLANG_PATH'               = 'C:\Program Files\LLVM\bin'
+  'CMAKE_POLICY_VERSION_MINIMUM' = '3.5'
+  # FFMPEG_DIR is only needed for the `amf-qsv` feature (libavcodec). The RTX box builds
+  # `--features nvenc`, which does NOT link FFmpeg, so it is intentionally left unset.
+}
+foreach ($k in $vars.Keys) {
+  [Environment]::SetEnvironmentVariable($k, $vars[$k], 'Machine')
+  Write-Host ("set (Machine)  {0} = {1}" -f $k, $vars[$k])
+}
+Write-Host "Done. Open a fresh shell (or reboot) for these to be inherited by new processes."