Add json_document and json_view: parse, accept, types, materialize

The public classes of the zero-copy view (#5295), in the new header
<nlohmann/json_view.hpp>:

- basic_json_document<BasicJsonType>: parse (borrowing contiguous byte
  inputs, owning rvalue strings, streams, and other inputs), parse_copy,
  accept, read, root, is_discarded, source, owns_source, node_count,
  memory_usage, shrink_to_fit
- basic_json_view<BasicJsonType>: type and the is_* queries, size, empty,
  materialize (the value parse() would produce, built by the same SAX
  handler), source_offset
- the aliases json_document, json_view, ordered_json_document, and
  ordered_json_view

A parse error throws the exception basic_json::parse would throw for the
same input: the library parser is run on the failing input, so messages,
positions, and exception ids are the same. Inputs of 4 GiB or more are
rejected with out_of_range.416.

The single header single_include/nlohmann/json_view.hpp keeps including
json.hpp; make amalgamate, check-amalgamation, include.zip, and release
handle it.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/view/number.hpp

@@ -0,0 +1,67 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+#pragma once
+
+#include <cstddef> // size_t
+#include <string> // string
+
+#include <nlohmann/json.hpp>
+#include <nlohmann/detail/view/macro_scope.hpp>
+#include <nlohmann/detail/view/node.hpp>
+
+NLOHMANN_JSON_NAMESPACE_BEGIN
+namespace detail
+{
+namespace view
+{
+
+/*!
+@brief the value of the float token of a node, as parse() converts it
+
+Uses the lexer's conversion (detail::convert_float), so that the values are
+bit-identical to parse(): float and double are converted without allocation
+and independent of the locale. The digit layout recorded while parsing locates
+the decimal point and the exponent without scanning the token.
+*/
+template<typename FloatType>
+NLOHMANN_VIEW_NOINLINE FloatType float_value(const char* first, const node& n)
+{
+    const char* const last = first + n.len;
+    const std::size_t neg = first[0] == '-' ? 1 : 0;
+    const std::size_t int_digits = n.extra & 0xFFu;
+    const std::size_t frac_digits = n.extra >> 8u;
+    std::size_t dot = std::string::npos;
+    std::size_t mantissa_end = n.len;
+    if (int_digits != 255 && frac_digits != 255)
+    {
+        dot = frac_digits != 0 ? neg + int_digits : std::string::npos;
+        mantissa_end = neg + int_digits + (frac_digits != 0 ? 1 + frac_digits : 0);
+    }
+    else
+    {
+        // more digits than the layout records: locate them
+        for (std::size_t i = 0; i < n.len; ++i)
+        {
+            if (first[i] == '.')
+            {
+                dot = i;
+            }
+            else if (first[i] == 'e' || first[i] == 'E')
+            {
+                mantissa_end = i;
+                break;
+            }
+        }
+    }
+    return convert_float<FloatType>(first, last, dot, mantissa_end);
+}
+
+}  // namespace view
+}  // namespace detail
+NLOHMANN_JSON_NAMESPACE_END