[CoreMidi] Make MidiUniversalMessage blittable (no explicit layout).

Explicit struct layout (with overlapping union fields) makes a struct
non-blittable, which forces the runtime to generate marshaling code and
prevents passing the struct by value through a 'delegate* unmanaged'
function pointer without overhead.

Rewrite MidiUniversalMessage and its variant structs to use sequential
blittable layout with opaque storage fields, exposing the native unions
through unsafe accessor properties that reinterpret the storage. Also
replace fixed buffers / array fields with named byte fields.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: rolfbjarne