Self-install into canonical layout on both platforms, --no-copy to opt out

Windows: --install-service creates %ProgramFiles%\gpu-turnstile and
%ProgramData%\gpu-turnstile, copies the exe and (if absent) the env
file in, and registers the copy. Linux: binary goes to
/var/lib/gpu-turnstile (not /usr/local/sbin: replacing a running binary
needs directory write, which must not be granted on a shared system dir
to a sandboxed service). --no-copy registers the current location
as-is on both platforms.

README.md

@@ -96,31 +96,31 @@ go build ./cmd/gpu-turnstile
 
 ### Run natively on Windows (current primary deployment)
 
-Download `gpu-turnstile.exe` from a release, put a `gpu-turnstile.env`
-next to it, and run it — or install it as a Windows service from an
-elevated shell:
+Download `gpu-turnstile.exe` from a release and install it as a Windows
+service from an elevated shell — the installer creates the canonical
+layout, copies the binary and (if none exists yet) your
+`gpu-turnstile.env` into it, and registers the copy:
 
 ```sh
-gpu-turnstile.exe --install-service        # auto-start service, recovery = restart
+gpu-turnstile.exe --install-service        # installs into Program Files, auto-start
+gpu-turnstile.exe --install-service --no-copy   # register in place instead
 gpu-turnstile.exe --remove-service
 ```
 
-The service uses the config file (services have no convenient
-environment); set `LOG_FILE` in it since there is no console.
-
-Suggested layout: `C:\Program Files\gpu-turnstile\` for the exe and
-`gpu-turnstile.env`, logs under `C:\ProgramData\gpu-turnstile\` via
-`LOG_FILE`. The service always runs as the virtual account
-`NT SERVICE\gpu-turnstile` (low-privilege, per-service, no password); the
-installer automatically grants it write access to the install and log
-directories — nothing else to do.
+Layout: `C:\Program Files\gpu-turnstile\` holds the exe and
+`gpu-turnstile.env`, logs go to `C:\ProgramData\gpu-turnstile\` (set
+`LOG_FILE` in the env file — there is no console). The service always runs
+as the virtual account `NT SERVICE\gpu-turnstile` (low-privilege,
+per-service, no password); the installer automatically grants it write
+access to the install and data directories — nothing else to do.
 
 ### Run natively on Linux (systemd)
 
 The same binary works on Linux. Install it as a systemd service as root:
 
 ```sh
-gpu-turnstile --install-service            # writes + enables + starts the unit
+gpu-turnstile --install-service            # installs into /var/lib/gpu-turnstile, enables + starts
+gpu-turnstile --install-service --no-copy  # register in place instead
 gpu-turnstile --remove-service
 ```