Replace with a measured STOPA head, not the unreproducible reference artefact

The previous stopa.pt was the retired reference implementation: it does not load with
the released code and no number was ever claimed for it.

This one was fitted and evaluated by the released code on the full published protocol
-- 48,024 EET fit rows, 33,200 enrolment, 629,800 probe utterances, conditions pooled,
split seed 42 / fit seed 0 -- and measured unknown-attack EER 9.3303% against a
published 16.43%. Recorded in results/stopa_measured.json.

Note this fits on STOPA's EET split, whereas the 16.43% baseline is a zero-shot system.

stopa.pt

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:60afdf17f2bc964791ff43f602e309870d016abb838a99d73173672cf20ec18e
-size 17934377
+oid sha256:9bedbd1b7c794a7e851482d7a5af701bda992d1bd0d5d5c5d8c1a6d98006b010
+size 18157599