feat(windows-client): SDL3 gamepads + docs — full stage-1 parity, MSVC-green

Adds the SDL3 gamepad service (near-verbatim port of the GTK client's — SDL3 is
cross-platform) and wires it into the winit app: per-session capture (buttons/axes,
DualSense touchpad + motion 0xCC), feedback (rumble, lightbar, raw DualSense effects),
single-pad-forwarded model with auto pad-type from the physical controller. Built from
source on Windows (no system SDL3).

- gamepad.rs: GamepadService (app-lifetime SDL thread) attach/detach on session
  connect/end; auto_pref resolves "Automatic" to the attached pad's type.
- app.rs: hold the service, attach on Connected, detach on Ended/Failed/close. Also
  simplify the keydown path (drop the identical if/else arms).
- main.rs: start the service for the windowed path, resolve GamepadPref from settings +
  the physical pad.

Build gotcha documented + fixed in the dev loop: SDL3's build-from-source MSVC
precompiled-header chokes on the `ü` in the dev box's username embedded in the cargo
registry path (MSB8084/C4828) — CARGO_HOME must be an ASCII path
(C:\Users\Public\.cargo). Unrelated to our code.

Docs: CLAUDE.md M4 + docs/windows-client-bootstrap.md status banner (winit-not-Reactor
rationale, CARGO_HOME gotcha, what's pending) + docs-site clients.md "Windows desktop
client (in development)". Crate is build + clippy + fmt + test green on
x86_64-pc-windows-msvc.

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

docs-site/content/docs/clients.md

@@ -54,6 +54,23 @@ connect straight away:
 punktfunk-client --connect <host>:9777   # skip the picker, start a session immediately
 ```
 
+## Windows desktop client (in development)
+
+`punktfunk-client` for Windows (`crates/punktfunk-client-windows`) is the native graphical client
+for Windows — pure Rust, the same `punktfunk/1` core as the Apple and Linux apps, with a winit +
+Direct3D11 present surface, WASAPI audio, FFmpeg decode, SDL3 controllers, network discovery, and
+PIN pairing. It builds on `x86_64-pc-windows-msvc` and runs the connect/decode/present/input path;
+hardware (D3D11VA) decode, 10-bit/HDR present, and a native host-list/settings window are in
+progress, so it is not yet packaged. For now it is driven from the command line:
+
+```sh
+punktfunk-client --discover                       # list hosts on the network
+punktfunk-client --connect <host>:9777            # stream (trust-on-first-use)
+punktfunk-client --connect <host>:9777 --pair 1234 # pair with the PIN the host shows
+```
+
+Until it ships, **Moonlight** remains the recommended way to stream to Windows (see below).
+
 ## Linux reference client (headless)
 
 `punktfunk-client-rs` (in the repo) is a command-line client for the native protocol, used for