feat(host/windows): ViGEm rumble back-channel + Windows clippy clean

Wire the host→client rumble path on Windows, the analogue of the Linux
uinput EV_FF read loop: a game's force-feedback on the virtual Xbox 360
pad is delivered by ViGEm's notification API (`request_notification` →
`spawn_thread`, gated by the crate's `unstable_xtarget_notification`
feature). A per-pad background thread stores the latest motor levels;
`pump_rumble` relays changes to the client on the universal 0xCA plane
(motors scaled 0..255 → 0..65535). Dropping the target aborts the
notification, so the thread exits with the session. Live verification
still needs a physical pad.

Also fix the Windows backends' clippy debt — these modules are cfg-
excluded from Linux CI, so `clippy -D warnings` never saw them, and the
VM's rustc 1.96 clippy is stricter on shared code than the CI image:
- dxgi: manual checked division → checked_div().map_or
- sendinput: `x = x | y` → `x |= y`
- sudovda: `.then(|| ptr)` → `.then_some(ptr)`
- m3 pick_compositor: drop the needless early return (match form)
- m3 resolve_compositor: Windows arm is a tail expr, not `return`

All Windows backends now build + clippy clean (default and --features
nvenc); Linux unaffected (fmt/clippy/check green).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/windows-host.md

@@ -27,13 +27,18 @@ Every OS-touching backend is implemented behind the existing traits and **builds
 | Capture (DXGI Desktop Duplication) | ✅ done | helpers unit-tested; DuplicateOutput needs a GPU-activated monitor |
 | NVENC (D3D11, `--features nvenc`) | ✅ compiles+links | needs a GPU at runtime |
 | Run host (serve/m3-host) | ✅ live | m3-host starts + listens; `c_abi_connection_roundtrip` passes |
-| Gamepad (ViGEm) | ✅ done | compiles; live needs ViGEmBus + a physical pad; rumble back-channel TODO |
+| Gamepad (ViGEm) | ✅ done | compiles incl. rumble back-channel; live needs ViGEmBus + a physical pad |
 | Host→client audio wiring | ✅ done | builds on MSVC; `m3` `audio_thread` active on Windows (silent VM → no samples to send) |
+| Rumble back-channel (ViGEm) | ✅ done | `request_notification` → background thread → 0xCA; live needs a physical pad |
 
 **Remaining for full parity:**
-- **ViGEm rumble back-channel** (`Xbox360Wired::request_notification`) — small; needs a physical pad.
-- **Live GPU/in-session validation** — SudoVDA monitor activation, DXGI capture, NVENC encode, and
-  SendInput injection all need a real GPU and an interactive (console) session, not SSH/Session-0.
+- **Live GPU/in-session validation** — SudoVDA monitor activation, DXGI capture, NVENC encode,
+  SendInput injection, and the ViGEm rumble path all need a real GPU and/or an interactive (console)
+  session + a physical pad, not SSH/Session-0.
+
+All Windows backends are `clippy -D warnings` and `rustfmt` clean on `x86_64-pc-windows-msvc` (the
+Windows-only modules are cfg-excluded from Linux CI, so run clippy on the VM after touching them — its
+rustc 1.96 clippy is stricter than the Linux CI image on shared code, e.g. `needless_return`).
 
 ### Building & testing on a real-GPU Windows box (NVENC)