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/errors.hpp

@@ -0,0 +1,81 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  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 <algorithm> // min
+#include <cstddef> // size_t
+#include <string> // string
+
+#include <nlohmann/json.hpp>
+#include <nlohmann/detail/view/builder.hpp>
+#include <nlohmann/detail/view/macro_scope.hpp>
+
+NLOHMANN_JSON_NAMESPACE_BEGIN
+namespace detail
+{
+namespace view
+{
+
+// Exceptions are thrown out of line, so that the accessors that may throw stay
+// small enough to be inlined.
+
+[[noreturn]] NLOHMANN_VIEW_NOINLINE inline void throw_type_error(int id, const char* prefix, const char* type)
+{
+    NLOHMANN_VIEW_THROW(type_error::create(id, concat(prefix, type), nullptr));
+}
+
+[[noreturn]] NLOHMANN_VIEW_NOINLINE inline void throw_out_of_range(int id, const std::string& msg)
+{
+    NLOHMANN_VIEW_THROW(out_of_range::create(id, msg, nullptr));
+}
+
+[[noreturn]] NLOHMANN_VIEW_NOINLINE inline void throw_invalid_iterator(int id, const char* msg)
+{
+    NLOHMANN_VIEW_THROW(invalid_iterator::create(id, msg, nullptr));
+}
+
+/*!
+@brief throw the exception BasicJsonType::parse would throw for this input
+
+The view accepts exactly the inputs parse() accepts, so on a failure the
+library parser is run on the same bytes: it throws the exception parse() would
+throw, with the same message, position, and "last read" token. The error path
+is cold, so this costs nothing on valid input. Should parse() accept the input
+nevertheless (a bug), the view's own failure is reported.
+*/
+template<typename BasicJsonType>
+[[noreturn]] NLOHMANN_VIEW_NOINLINE void throw_parse_failure(const parse_failure& f, const char* src, std::size_t size,
+        bool ignore_comments, bool ignore_trailing_commas)
+{
+    if (f.code == error_code::input_too_large)
+    {
+        NLOHMANN_VIEW_THROW(out_of_range::create(416, "input of 4 GiB or more is not supported by json_document", nullptr));
+    }
+    const BasicJsonType accepted = BasicJsonType::parse(src, src + size, nullptr, true, ignore_comments, ignore_trailing_commas);
+    static_cast<void>(accepted);
+
+    position_t pos;
+    const std::size_t off = (std::min)(f.offset, size);
+    pos.chars_read_total = off + 1;
+    std::size_t line_start = 0;
+    for (std::size_t i = 0; i < off; ++i)
+    {
+        if (src[i] == '\n')
+        {
+            ++pos.lines_read;
+            line_start = i + 1;
+        }
+    }
+    pos.chars_read_current_line = off + 1 - line_start;
+    NLOHMANN_VIEW_THROW(parse_error::create(101, pos, "syntax error while parsing value", nullptr));
+}
+
+}  // namespace view
+}  // namespace detail
+NLOHMANN_JSON_NAMESPACE_END