AK: Add FixedBitmap template class for stack-allocated bitmaps

This is a simple bitmap class that stores its data in a fixed-size
array on the stack, avoiding heap allocation for small, known-size
bitmaps.

AK/FixedBitmap.h

@@ -0,0 +1,50 @@
+/*
+ * Copyright (c) 2026, Andreas Kling <andreas@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Array.h>
+#include <AK/StdLibExtras.h>
+#include <AK/Types.h>
+
+namespace AK {
+
+template<size_t Size>
+class FixedBitmap {
+public:
+    constexpr FixedBitmap(bool default_value)
+    {
+        fill(default_value);
+    }
+
+    constexpr void fill(bool value)
+    {
+        __builtin_memset(m_data.data(), value ? 0xff : 0x00, size_in_bytes());
+    }
+
+    constexpr void set(size_t index, bool value)
+    {
+        VERIFY(index < Size);
+        if (value)
+            m_data[index / 8] |= static_cast<u8>(1u << (index % 8));
+        else
+            m_data[index / 8] &= static_cast<u8>(~(1u << (index % 8)));
+    }
+
+    [[nodiscard]] constexpr bool get(size_t index) const
+    {
+        VERIFY(index < Size);
+        return 0 != (m_data[index / 8] & (1u << (index % 8)));
+    }
+
+    [[nodiscard]] constexpr size_t size() const { return Size; }
+    [[nodiscard]] constexpr size_t size_in_bytes() const { return ceil_div(Size, static_cast<size_t>(8)); }
+
+private:
+    Array<u8, ceil_div(Size, static_cast<size_t>(8))> m_data;
+};
+
+}