LibWeb: Implement some of SourceBuffer.appendBuffer()

Currently, it just fires the error event immediately.

Libraries/LibWeb/MediaSourceExtensions/SourceBufferProcessor.h

@@ -0,0 +1,57 @@
+/*
+ * Copyright (c) 2026-present, the Ladybird developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/AtomicRefCounted.h>
+#include <AK/ByteBuffer.h>
+#include <AK/Function.h>
+
+namespace Web::MediaSourceExtensions {
+
+// https://w3c.github.io/media-source/#sourcebuffer-append-state
+enum class AppendState : u8 {
+    WaitingForSegment,
+    ParsingInitSegment,
+    ParsingMediaSegment,
+};
+
+enum class AppendMode : u8 {
+    Segments,
+    Sequence,
+};
+
+class SourceBufferProcessor : public AtomicRefCounted<SourceBufferProcessor> {
+public:
+    SourceBufferProcessor();
+    ~SourceBufferProcessor();
+
+    bool updating() const { return m_updating; }
+    void set_updating(bool value) { m_updating = value; }
+
+    AppendMode mode() const;
+    bool is_buffer_full() const;
+    using AppendErrorCallback = Function<void()>;
+
+    void set_append_error_callback(AppendErrorCallback);
+
+    void append_to_input_buffer(ReadonlyBytes);
+
+    void run_segment_parser_loop();
+    void reset_parser_state();
+    void run_coded_frame_eviction();
+
+private:
+    bool m_updating { false };
+    ByteBuffer m_input_buffer;
+
+    AppendErrorCallback m_append_error_callback;
+    AppendState m_append_state { AppendState::WaitingForSegment };
+    AppendMode m_mode { AppendMode::Segments };
+    bool m_buffer_full_flag { false };
+};
+
+}