refactor(host/windows): clean up DDA path + add a proper Windows service

Final cleanup after the DDA-parity work, plus an end-user service to replace the
PsExec/VBS/scheduled-task launch chain.

Cleanup (behavior-preserving):
- sudovda.rs: drop the dead legacy GDI isolate_displays/restore_displays (CCD is
  the sole isolation path), the always-empty Monitor.isolated field, and the
  vestigial reassert_isolation + PUNKTFUNK_ISOLATE_DISPLAYS knob; fix stale comments.
- dxgi.rs: downgrade leftover debug warns/infos (DuplicateOutput1 retry, FALLBACKS,
  hook-hits, AcquireNextFrame idle timeout) to debug!; remove the PUNKTFUNK_NO_CURSOR
  per-frame test knob.

Windows service (src/service.rs, `punktfunk-host service`):
- SCM supervisor (windows-service crate) that duplicates its LocalSystem token,
  retargets it to the active console session, and CreateProcessAsUserW's the host
  there (Sunshine/Apollo model) — relaunching on exit and console session switch,
  inside a kill-on-close job object so a service crash never orphans the host.
- install/uninstall/start/stop/status subcommands: one elevated `service install`
  registers an auto-start LocalSystem service + firewall rules + a default host.env.
- Config moves to %ProgramData%\punktfunk\host.env; config_dir() now resolves to
  %ProgramData%\punktfunk on Windows (replacing the APPDATA=C:\Users\Public hack),
  with a PUNKTFUNK_CONFIG_DIR override. Logs land in %ProgramData%\punktfunk\logs\.
- merged_env_block (shared with the WGC helper) now also carries RUST_LOG.
- docs/windows-service.md + scripts/windows/host.env.example; windows-host.md updated.

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

docs/windows-service.md

@@ -0,0 +1,93 @@
+# Windows service (deployment)
+
+The `PunktfunkHost` Windows service is the end-user way to run the host on Windows. It replaces the
+manual bring-up chain (a scheduled task → `PsExec64 -s -i 1` → `wscript launch.vbs` → `host-run.cmd`)
+with one command, auto-start on boot, and supervision.
+
+## Install
+
+From an **elevated** (Administrator) prompt:
+
+```powershell
+punktfunk-host service install   # register auto-start LocalSystem service + firewall rules + default host.env
+punktfunk-host service start     # start it now (also starts automatically on every boot)
+```
+
+`service install` is idempotent — run it again after upgrading the exe to re-point the service at the
+new binary. Register whatever location you keep the exe in (e.g. `C:\Program Files\punktfunk\`); the
+service records the current exe path.
+
+Other subcommands:
+
+```powershell
+punktfunk-host service stop
+punktfunk-host service status
+punktfunk-host service uninstall   # stop + delete the service + remove its firewall rules
+```
+
+## How it works
+
+The host must run **as SYSTEM in the interactive session** (Session 1+): Desktop Duplication of the
+secure desktop (UAC / lock / login) and `SendInput` need SYSTEM, and capture/injection need the
+interactive session, which a plain Session-0 service is not in.
+
+So the service (itself in Session 0) **never captures**. On start, and whenever the active console
+session changes, it:
+
+1. resolves the active console session (`WTSGetActiveConsoleSessionId`),
+2. duplicates its own LocalSystem token and retargets it to that session (`SetTokenInformation`
+   `TokenSessionId`),
+3. launches the host there with `CreateProcessAsUserW` (`lpDesktop = winsta0\default`),
+4. supervises it: relaunches on exit/crash (with backoff) and on a console connect/disconnect.
+
+A kill-on-close **job object** ensures a service crash never orphans the SYSTEM host. The host in turn
+spawns the WGC helper into the *user* session (see [`windows-secure-desktop.md`](windows-secure-desktop.md))
+— two nested launches. Lock/unlock are handled inside the host (the `DesktopWatcher` DDA↔WGC mux), so
+the service deliberately does **not** relaunch on lock/unlock — only on a real session switch.
+
+This is the same model Sunshine/Apollo use.
+
+## Configuration
+
+Config lives in **`%ProgramData%\punktfunk\host.env`** (KEY=VALUE lines, `#` comments). `service
+install` writes a default if none exists. Template: [`scripts/windows/host.env.example`](../scripts/windows/host.env.example).
+
+```ini
+PUNKTFUNK_ENCODER=nvenc
+PUNKTFUNK_VIDEO_SOURCE=virtual
+PUNKTFUNK_SECURE_DDA=1
+RUST_LOG=info
+# PUNKTFUNK_HOST_CMD=serve --native    # the host subcommand the service launches (default)
+```
+
+The service loads these into its environment and carries `PUNKTFUNK_*` + `RUST_LOG` to the host child
+(the same env-merge the WGC helper uses). Restart the service after editing:
+
+```powershell
+punktfunk-host service stop; punktfunk-host service start
+```
+
+The host's identity (cert/pairing/mgmt token/library) also lives under `%ProgramData%\punktfunk` — a
+machine-wide dir the SYSTEM service and the interactive user share, surviving user logout.
+`PUNKTFUNK_CONFIG_DIR` overrides the location (both platforms; handy for tests).
+
+## Logs
+
+- `%ProgramData%\punktfunk\logs\service.log` — the service's own supervision log (spawn/exit/session
+  switches).
+- `%ProgramData%\punktfunk\logs\host.log` — the host child's stdout/stderr.
+
+## Prerequisites
+
+- The host built with `--features nvenc` for NVENC (the driver ships `nvEncodeAPI64.dll`; no SDK
+  needed at runtime). Software encode otherwise.
+- The **SudoVDA** indirect display driver installed (for `PUNKTFUNK_VIDEO_SOURCE=virtual`).
+- **ViGEmBus** for virtual gamepads (optional).
+
+## Gotchas
+
+- `service install`/`uninstall` need an **elevated** prompt (the SCM rejects non-admin).
+- `service run` is the SCM entry point — don't run it by hand (it errors with a hint).
+- A **graceful** stop currently `TerminateProcess`es the host, so its RAII teardown (SudoVDA monitor
+  REMOVE) doesn't run; a stale virtual monitor can linger until the next start. A cooperative-stop
+  signal is a follow-up.