refactor(host/W6.0): hoist GamepadEvent/GamepadFrame to punktfunk_core::input

First de-coupling for the host crate carve (plan §W6.0 / §2.4): the GameStream
(Moonlight-plane) decoded controller types were defined in gamestream/gamepad.rs — the
"junk drawer" — yet consumed 18× by the platform-neutral input injectors AND by the
Moonlight decode path. Once inject becomes pf-inject, reaching them via crate::gamestream
would be an illegal upward edge. Move the two types to core::input (below both planes;
inject already depends on core) and repoint every consumer. Also consolidate the
duplicated MAX_PADS onto the existing core::input::MAX_PADS. The gamestream BTN_* const
aliases stay for now (separate follow-up); decode()/rumble/tests remain in the Moonlight
plane, now importing the types from core.

Verified: Linux (home-worker-5) clippy -p punktfunk-core -p punktfunk-host --all-targets
-D warnings + gamepad tests green; Windows (192.168.1.158) clippy -p punktfunk-host
--features nvenc,amf-qsv --all-targets green (the inject/windows/* consumers compile).

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

crates/punktfunk-host/src/inject/windows/dualshock4_windows.rs

@@ -187,7 +187,7 @@ impl PadProto for Ds4WinProto {
 
     /// Merge buttons/sticks/triggers from the frame, preserving touch + motion + pad clicks (rich-
     /// plane fields that must survive a button-only frame) — exactly as `linux/dualshock4.rs` does.
-    fn merge_frame(&self, prev: &DsState, f: &crate::gamestream::gamepad::GamepadFrame) -> DsState {
+    fn merge_frame(&self, prev: &DsState, f: &punktfunk_core::input::GamepadFrame) -> DsState {
         // Steam back grips have no DS4 slot — fold them onto standard buttons per the configured
         // policy (default drop) so they aren't silently lost.
         let buttons = crate::inject::steam_remap::fold_paddles(f.buttons, self.remap.paddles);