[F52.1] mxaccess-codec: BytesMut output buffer for write encoder

Adds `write_message::encode_to_bytes_mut` (and the timestamped variant)
returning a freshly-allocated `BytesMut`. Allocation count is identical
to `encode` (2 allocs/op for fixed-width scalars); the benefit is
downstream — consumers can `BytesMut::split_to` / `freeze` and forward
the body bytes to a wire-level sink without an intermediate copy.

The body builders (`encode_boolean` / `encode_fixed` / `encode_variable`
/ `encode_array`) were refactored to fill a pre-sized `&mut [u8]`
rather than each allocating their own `Vec<u8>`. The dispatcher
computes the body size up front via small `*_body_size` helpers and
resizes the destination buffer (Vec or BytesMut) once. This is also
the prerequisite refactor for F52.3.

Bench delta in `design/M6-bench-baseline.md` § F52.1; existing
`encode` row unchanged at 2 allocs/op. All 265 round-trip tests
unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

rust/Cargo.lock

@@ -693,6 +693,7 @@ dependencies = [
 name = "mxaccess-codec"
 version = "0.0.0"
 dependencies = [
+ "bytes",
  "thiserror 2.0.18",
 ]