Tighten upper-bound debug_assert in bitfield_unit accessors

The upper-bound check in get/set/raw_get/raw_set and their const-generic
siblings used floor division: `(bit_offset + bit_width) / 8 <= len`.
The byte-loop touches indices up to
`start_byte + bytes_needed - 1 = ceil((bit_offset + bit_width) / 8) - 1`,
so the correct check is `(bit_offset + bit_width + 7) / 8 <= len`.

Codegen never produces values that would fail the tighter assert, so
this is a hardening of the safety check rather than a runtime fix.
Regenerate the matching expectations.

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

Author: 2 people