to_bjdata() emits the Draft-3-only 'B' (byte) marker for _ArrayType_:"byte" even in default Draft-2 mode, and the value does not round-trip #5404

Description

The BJData ndarray writer maps _ArrayType_ = "byte" to the type marker B unconditionally, with no gate on bjdata_version. But B (byte) is a Draft-3-only marker: the docs (bjdata.md, "Binary values") state it "must be explicitly enabled using the version parameter of to_bjdata", and BJData Draft 2 has no B type.

In write_bjdata_ndarray (include/nlohmann/detail/output/binary_writer.hpp) the type map contains {"byte", 'B'} and the marker is written regardless of the requested version. So with the default version = draft2, an annotated byte array produces [$B#…, a stream that is invalid under Draft 2 and that a strict third-party Draft-2 reader will reject.

Separately, unlike every other _ArrayType_ (e.g. uint8, which round-trips back to the annotated object), the result parses back as a binary value, so to_bjdata/from_bjdata changes the value.

Low severity / spec-conformance and interop, and only reachable with JData-annotated input, but the behavior contradicts the documented version-gating of B.

Reproduction steps

Serialize a byte ndarray with default (Draft-2) options and inspect the bytes / round-trip.

Expected vs. actual results

• Expected (Draft-2 default): either a fallback encoding that does not use the Draft-3 B marker, or an error; and round-trip equality like the other dtypes.
• Actual: bytes contain B (0x42); the value round-trips to a binary value instead of the original object.

Minimal code example

#include <nlohmann/json.hpp>
#include <cstdio>
using json = nlohmann::json;

int main()
{
    json j;
    j["_ArrayType_"] = "byte";
    j["_ArraySize_"] = json::array({2});
    j["_ArrayData_"] = json::array({1, 2});
    auto v = json::to_bjdata(j);              // default draft2
    for (auto b : v) std::printf("%02x ", b); // 5b 24 42 23 5b 69 02 5d 01 02  ("[$B#[i2]..")
    std::printf("\nback=%s\n", json::from_bjdata(v).dump().c_str());
}

Error messages

5b 24 42 23 5b 69 02 5d 01 02
back={"bytes":[1,2],"subtype":null}

(The stream contains the Draft-3 B marker under the Draft-2 default, and the round-trip yields a binary value rather than the original annotated object.)

Suggested fix

Only accept "byte"/emit B when bjdata_version selects Draft 3; otherwise fall back to a plain-object encoding (consistent with the other unrepresentable-annotation cases).

Compiler and operating system

g++ 13.3.0 (Ubuntu 24.04, x86-64)

Library version

develop @ 01853ed6bcf9ebe88ec2e248ea757b86417b1487

Validation

• The bug also occurs if the latest version from the develop branch is used.
• I can successfully compile and run the unit tests.