Forward rumble back to the guest controller (both backends)

The XInput hook used to swallow XInputSetState; now it records the requested
left/right motor speeds into the status back-channel (protocol v8->v9: per-slot
rumble_left/right in HookStatus). Each frame the host reads them and, only on
change, drives the guest's actuator via the active backend:
- XInput: XInputSetState on the guest's slot (the open question is whether Steam's
  RPT virtual pad accepts vibration and routes it to the guest -- needs live RPT);
- Steam Input: SteamInput TriggerVibration on the slot's controller handle, with the
  XInput fallback for slots Steam isn't driving.

InputSource gains a set_rumble(slot,left,right) hook (default no-op) implemented by
both backends; SteamInputSource now tracks per-slot controller handles + which slots
it drives.

Verified: hook_selftest (x64 + x86) now asserts the hook records the rumble from
XInputSetState into the status; full build x64 + x86 clean; ctest x64 9/9, x86 3/3.

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

README.md

@@ -76,14 +76,6 @@ is removed from this list once done — so the top item is always next. The
 self-verifiable tooling / UI / input items come first; the game-pipeline items that
 need a real game (and Remote Play) to fully validate come last.
 
-- **Rumble / haptics forwarding (both backends).** Currently unsupported — the XInput
-  hook swallows `XInputSetState`. Add a reverse path: the hook captures the game's
-  `XInputSetState` (left/right motor) and publishes it over a hook→host channel (the
-  back-channel already exists), and the host drives the guest's actuators per backend —
-  **XInput:** call `XInputSetState` on the guest's slot (the viability unknown is
-  whether Steam's RPT virtual pad accepts vibration and routes it to the guest);
-  **Steam Input:** `SteamInput()->TriggerVibration` / `Legacy_TriggerHapticPulse`.
-  Map each guest slot to the right actuator.
 - **Per-backend input debug visualization.** To separate "wrong input *into* the
   tool" from "wrong input *out to* the game", show three distinct views in the
   Controllers panel (under Debug details): (a) **received via XInput** (raw