Encode std::byte ranges compactly and read fixed char arrays in BEVE

[stephenberry]

Fixes #2647.

Problem

The BEVE codec mishandled arrays of two primitive element types:

1. std::array<char, N> failed to compile on read. It is classified as str_t (via array_char_t), and from<BEVE, str_t> unconditionally called value.resize(n), which std::array does not have.
2. std::vector<std::byte> / std::array<std::byte, N> used ~2× the space. std::byte is a scoped enum (not num_t), so it fell through to the generic_array branch — a one-byte type header per element. This was inconsistent with std::vector<uint8_t> (a compact u8 typed array) and with a scalar std::byte, which was already encoded as a u8 number.

Fix

• New beve_num_t = num_t<T> || std::same_as<T, std::byte> concept opts std::byte into the numeric typed-array dispatch in write / read / size. The existing numeric formulas already yield byte_count == 0 and an unsigned u8 header for std::byte, so the wire output is byte-for-byte identical to uint8_t (e.g. a 16-element vector goes 34 → 18 bytes). std::byte and uint8_t ranges are now cross-readable in both directions.
• The generic-enum reader gains a tagged overload that delegates to its underlying integer reader, so the typed-array conversion paths work for std::byte.
• from<BEVE, str_t> now stores into resizable strings, string views, and fixed std::array<char, N> (bounds-checked, remainder zero-filled) through a shared helper used by both the tagged and untagged read paths.

Wire-driven paths (skip, beve_to_json, ptr) need no change — they handle the resulting u8 typed array purely from the tag.

Wire-format note

This intentionally changes the BEVE encoding of std::byte arrays. Data written by older versions using the generic-array form is rejected cleanly with error_code::syntax_error (no UB). This aligns std::byte arrays with both scalar std::byte and std::vector<uint8_t>.

Tests

11 regression tests added to beve_test.cpp covering: compact encoding + wire-equality with uint8_t, cross-readability, std::array<std::byte, N>, empty and non-contiguous (std::deque) byte ranges, std::byte struct members, beve_to_json rendering, full and short (zero-filled) std::array<char, N> round trips, oversized-payload rejection, and untagged mode. Full beve_test suite passes (381 tests), along with lazy_beve_test and skip_null_on_read_test.