Correctly scale batches for FACTOR_FINDER mode

This release calculates the first valid sieving element after wheel factorization in FACTOR_FINDER Quadratic Sieve mode, takes this as overall additive offset on all other work items, and multiplies just the batch number by the batch size. (Getting the order of operations right on wheel factorization interval scaling has always been the trickiest part, since the original development of PRIME_PROVER mode, as well.)

Full Changelog: v6.4.1...v6.4.2

sha1sum results:
44bffee0e3e215d686c4af364c254c824583d4ed FindAFactor-6.4.2-cp310-cp310-manylinux_2_35_x86_64.whl
f8387986c0ffd0fb0421acbf28de2e725a9a37c9 FindAFactor-6.4.2-cp312-cp312-manylinux_2_39_x86_64.whl
9ade956cd0188882baabe6581bf92fe49066e28a FindAFactor-6.4.2-cp312-cp312-win_amd64.whl
1e82aa0faa5ec15b10b7ace130d84dc11a2acf8e FindAFactor-6.4.2-cp313-cp313-macosx_13_0_x86_64.whl
561b683ea0581198bce23ec4fd44a09fcbb89f0e FindAFactor-6.4.2-cp313-cp313-macosx_14_0_arm64.whl
b1e20d6e1c1b4d0f3e6eff223242a0aa9a81e29b FindAFactor-6.4.2-cp313-cp313-macosx_15_0_arm64.whl
c50ec2804e9750eec7c12e4760194e6c0d0f1520 FindAFactor-6.4.2-cp38-cp38-manylinux_2_31_x86_64.whl
a164535c1caa53842b34cac68cb7a1f838503289 findafactor-6.4.2.tar.gz