fix(agent): make native H.264 viewer render live frames

The native viewer's H.264 path (Task 7 first-cut, compile-verified only)
never rendered a frame. Three stacked bugs, all confirmed via live loopback:

1. decoder: MF_E_NOTACCEPTING (0xC00D36B5) was treated as fatal and only
   one output was drained per call, so once the MFT filled it rejected
   every subsequent frame. decode() now returns Vec<DecodedFrame>, drains
   on back-pressure and retries the unconsumed sample, then drains all
   ready outputs.
2. decoder: the NV12 output type was hand-built and rejected by the MS
   H.264 decoder MFT (MF_E_TRANSFORM_TYPE_NOT_SET, 0xC00D6D60). It is now
   negotiated by enumerating GetOutputAvailableType on STREAM_CHANGE /
   TYPE_NOT_SET.
3. render: a manual pump_messages() in about_to_wait stole winit's own
   thread messages and froze the event loop after one iteration, so frames
   were never drained from the channel. Removed; winit's run_app pump
   already services the WH_KEYBOARD_LL hook.

Validated on a 5070 loopback: 0 decode errors, frames decode/paint/present
(present count 0 -> 1740). Reviewed (APPROVE-WITH-NITS); diagnostics stripped.

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

agent/src/viewer/input.rs

@@ -27,9 +27,8 @@ use tracing::trace;
 use windows::{
     Win32::Foundation::{LPARAM, LRESULT, WPARAM},
     Win32::UI::WindowsAndMessaging::{
-        CallNextHookEx, DispatchMessageW, PeekMessageW, SetWindowsHookExW, TranslateMessage,
-        UnhookWindowsHookEx, HHOOK, KBDLLHOOKSTRUCT, LLKHF_EXTENDED, MSG, PM_REMOVE,
-        WH_KEYBOARD_LL, WM_KEYDOWN, WM_KEYUP, WM_SYSKEYDOWN, WM_SYSKEYUP,
+        CallNextHookEx, SetWindowsHookExW, UnhookWindowsHookEx, HHOOK, KBDLLHOOKSTRUCT,
+        LLKHF_EXTENDED, WH_KEYBOARD_LL, WM_KEYDOWN, WM_KEYUP, WM_SYSKEYDOWN, WM_SYSKEYUP,
     },
 };
 
@@ -237,18 +236,6 @@ fn current_modifiers() -> proto::Modifiers {
     }
 }
 
-/// Pump Windows message queue (required for hooks to work).
-#[cfg(windows)]
-pub fn pump_messages() {
-    unsafe {
-        let mut msg = MSG::default();
-        while PeekMessageW(&mut msg, None, 0, 0, PM_REMOVE).as_bool() {
-            let _ = TranslateMessage(&msg);
-            DispatchMessageW(&msg);
-        }
-    }
-}
-
 // Non-Windows stubs
 #[cfg(not(windows))]
 #[allow(dead_code)]
@@ -262,10 +249,6 @@ impl KeyboardHook {
     }
 }
 
-#[cfg(not(windows))]
-#[allow(dead_code)]
-pub fn pump_messages() {}
-
 #[cfg(test)]
 mod tests {
     use super::*;