wasapi: address buffer overflow when using -wasapishared (#758)

## Link to GitHub Issue or related Pull Request, if one exists
Building on #745 

## Description of change
With `-wasapishared`, some users see `AUDCLNT_E_BUFFER_TOO_LARGE`
(`0x88890006`).

The existing one-device-period buffer clamp wasn't enough on endpoints
with a small shared buffer; the shared buffer is double-buffered, so a
full-buffer write often exceeds the free space.

### Fix

Added a FIFO bridge to `SharedRedirect` that decouples the game's
per-event writes from the shared engine's clock:

- **`GetBuffer`** hands the game a pointer into the FIFO tail to write
in place.
- **`ReleaseBuffer`** commits the write and drains `min(pending,
device_free)` frames to the device - so a write can never exceed what
the device accepts, structurally preventing the overflow on any
endpoint.
- **`GetCurrentPadding`** reports the FIFO fill level (capped to the
reported buffer size) so poll/timer-driven games pace correctly against
the virtual buffer.

## Testing

src/spice2x/hooks/audio/backends/wasapi/shared.cpp

@@ -1,10 +1,13 @@
 #include "shared.h"
 
+#include <algorithm>
+
 #include <audioclient.h>
 
 #include "hooks/audio/audio.h"
 #include "util/logging.h"
 
+#include "util.h"
 #include "defs.h"
 
 namespace hooks::audio {
@@ -58,8 +61,9 @@ namespace hooks::audio {
     }
 
     UINT32 SharedRedirect::clamp_buffer_size(IAudioClient *real, uint32_t sample_rate,
-            UINT32 device_frames) const {
+            UINT32 device_frames) {
         if (!this->redirected_from_exclusive || real == nullptr || sample_rate == 0 || device_frames == 0) {
+            this->reported_frames = device_frames;
             return device_frames;
         }
 
@@ -69,10 +73,115 @@ namespace hooks::audio {
         if (SUCCEEDED(real->GetDevicePeriod(&period, nullptr)) && period > 0) {
             const UINT32 period_frames = (UINT32) ((period * sample_rate) / 10000000);
             if (period_frames > 0 && period_frames < device_frames) {
+                this->reported_frames = period_frames;
                 return period_frames;
             }
         }
 
+        this->reported_frames = device_frames;
         return device_frames;
     }
+
+    void SharedRedirect::enable_bridge(int frame_bytes) {
+        if (!this->redirected_from_exclusive || frame_bytes <= 0) {
+            return;
+        }
+
+        this->frame_bytes = frame_bytes;
+        this->device_buffer_frames = 0;
+        this->fifo.clear();
+
+        log_info("audio::wasapi", "shared-mode buffer bridge enabled (frame size {} bytes)",
+                frame_bytes);
+    }
+
+    BYTE *SharedRedirect::begin_write(UINT32 frames) {
+        // reserve space at the FIFO tail and let the game write straight into it - no scratch copy.
+        this->pending_write_offset = this->fifo.size();
+        this->fifo.resize(this->pending_write_offset + (size_t) frames * this->frame_bytes);
+
+        return this->fifo.data() + this->pending_write_offset;
+    }
+
+    void SharedRedirect::commit_write(UINT32 frames, bool silent) {
+        // trim the tail reservation to the frames actually written; zero it in place if silent.
+        const size_t end = this->pending_write_offset + (size_t) frames * this->frame_bytes;
+        if (silent) {
+            std::fill(this->fifo.begin() + this->pending_write_offset,
+                    this->fifo.begin() + end, (BYTE) 0);
+        }
+        this->fifo.resize(end);
+    }
+
+    UINT32 SharedRedirect::pending_frames() const {
+        if (this->frame_bytes <= 0) {
+            return 0;
+        }
+
+        return (UINT32) (this->fifo.size() / this->frame_bytes);
+    }
+
+    UINT32 SharedRedirect::virtual_padding() const {
+        const UINT32 pending = this->pending_frames();
+        return this->reported_frames > 0 ? std::min(pending, this->reported_frames) : pending;
+    }
+
+    HRESULT SharedRedirect::drain(IAudioRenderClient *real, IAudioClient *client,
+            const WAVEFORMATEXTENSIBLE &device_format, float boost) {
+        if (!this->bridge_enabled()) {
+            return S_OK;
+        }
+
+        // cache the real device buffer size once; it is fixed for the life of the stream.
+        if (this->device_buffer_frames == 0) {
+            if (FAILED(client->GetBufferSize(&this->device_buffer_frames))
+                    || this->device_buffer_frames == 0) {
+                return S_OK;
+            }
+        }
+
+        const UINT32 pending = this->pending_frames();
+        if (pending == 0) {
+            return S_OK;
+        }
+
+        // push only as many frames as the device currently has free, keeping the rest queued. this
+        // self-paces to the engine's real consumption so a full-buffer write never overflows.
+        UINT32 padding = 0;
+        if (FAILED(client->GetCurrentPadding(&padding))) {
+            return S_OK;
+        }
+        const UINT32 device_free = this->device_buffer_frames > padding
+                ? this->device_buffer_frames - padding
+                : 0;
+        if (device_free == 0) {
+            return S_OK;
+        }
+
+        const UINT32 to_write = std::min(pending, device_free);
+
+        BYTE *dev = nullptr;
+        HRESULT ret = real->GetBuffer(to_write, &dev);
+        if (FAILED(ret) || dev == nullptr) {
+            return ret;
+        }
+
+        const size_t bytes = (size_t) to_write * this->frame_bytes;
+        std::copy(this->fifo.begin(), this->fifo.begin() + bytes, dev);
+
+        // mute the first few buffers to avoid a startup pop, then apply the volume boost.
+        if (this->buffers_to_mute > 0) {
+            std::fill(dev, dev + bytes, (BYTE) 0);
+            this->buffers_to_mute--;
+        } else if (boost != 1.0f) {
+            apply_gain(dev, to_write, device_format, boost);
+        }
+
+        ret = real->ReleaseBuffer(to_write, 0);
+
+        // drop the frames just handed to the device from the front of the FIFO.
+        this->fifo.erase(this->fifo.begin(), this->fifo.begin() + bytes);
+
+        return ret;
+    }
 }