Fix std::byte and fixed std::array<char,N> handling in CBOR/MsgPack/BSON

[stephenberry]

Follow-up to #2649 (the BEVE fix for #2647), addressing the same family of issues in the other binary codecs. Found by reproducing each case against CBOR, MsgPack, and BSON.

Summary

	std::byte ranges	std::array<char,N> read
CBOR	❌ ambiguous (vector/span) / unreadable (array) → fixed	❌ compile error → fixed
MsgPack	✅ already compact (bin)	❌ undefined read → fixed
BSON	✅ already compact (binary)	❌ no viable operator= → fixed

CBOR

• std::vector<std::byte> / std::span<std::byte> were ambiguous (the byte-string specialization tied with the generic array specialization) and did not compile on write or read. The byte-string specs now require a contiguous std::byte range, and the generic array specs exclude contiguous std::byte ranges, so exactly one matches.
• std::array<std::byte,N> wrote a valid byte string but couldn't read it back. The byte-string reader is generalized to store into a fixed-size buffer (bounds-checked, remainder zero-filled), for both definite- and indefinite-length byte strings.
• std::array<uint8_t,N> had the same write/read asymmetry (a pre-existing gap surfaced during review): it wrote a byte string with no matching reader. Added a dedicated reader mirroring the existing std::vector<uint8_t> one. std::array<uint8_t,N> and std::vector<uint8_t> are now cross-readable.
• std::array<char,N> failed to compile on read (the text reader called resize/append/assign). The text reader now handles fixed char arrays.

MsgPack

• std::byte ranges were already encoded as the compact bin format. Only std::array<char,N> read was missing (write produced a string; read was an undefined template). Added a from<MSGPACK, array_char_t> reader.

BSON

• std::byte ranges were already compact binary. A std::array<char,N> string member failed to compile on read (no viable operator=); the string reader now handles fixed char arrays.

Semantics

Consistent with #2649: reading a payload longer than a fixed array yields error_code::syntax_error; a shorter payload is copied and the remainder zero-filled. Wire-driven paths and non-contiguous byte ranges (e.g. std::deque<std::byte> → CBOR array of numbers) are unchanged.

Tests

11 regression tests added (CBOR 6, MsgPack 3, BSON 3 — incl. the uint8_t array case): compact/byte-string encoding, cross-readability, fixed std::array<std::byte,N>/std::array<char,N> round trips, short-payload zero-fill, oversize-payload rejection, and struct members. All existing suites pass: CBOR 235, MsgPack 54, BSON 120.

Verification

Reviewed by an adversarial multi-agent pass (overload-resolution, memory-safety/bounds, round-trip semantics). No new defects; round-trips verified exact under ASan+UBSan. The std::array<uint8_t,N> reader was added in response to that review.