- pack_lanes: assert elem_w < 32 and pack the full element width instead
of silently dropping bits >= 31.
- Remove the dead `cell` struct member and its unused assignment in run()
(every emit helper shadows it with its own local `cell`).
- Decorrelate the pseudo-random bc bits from en (independent mix) so they
no longer share an LFSR bit (e.g. en[7]/bc[0] for n=8).
- Add purpose comments to fingerprint_dsel and lane_of_bit.
Declined the std::stoi/std::stoll arg-parsing suggestion: it matches the
established convention in sibling passes (opt_argmax, opt_priority_onehot).
Co-authored-by: Cursor <cursoragent@cursor.com>
