Fix Clippy loop-counter lint in solvate residue assignment

[Copilot]

Clippy fails in crates/bio-forge/src/ops/solvate.rs because res_id_counter was used as a manual loop counter when assigning IDs to generated water residues. This updates the loop to use idiomatic iterator zipping while preserving residue ID sequencing and insertion order.

• Root cause

  • Water residue IDs were assigned via a mutable external counter incremented inside for mut residue in new_waters.

• Code change

  • Replaced manual counter state with (1..).zip(new_waters) and destructured (res_id_counter, mut residue) in the loop.
  • Kept existing behavior for:
    • residue.id assignment
    • solvent_chain.add_residue(residue)
    • water_positions.push(res_id_counter)

• Representative diff

  for (res_id_counter, mut residue) in (1..).zip(new_waters) {
      residue.id = res_id_counter;
      solvent_chain.add_residue(residue);
      water_positions.push(res_id_counter);
  }