v10.21.0 contradiction judge gains an UNRELATED verdict

Fixed

• The phase-4 contradiction judge presumed its two inputs were about the
  same topic (the prompt asserted "these memories are about similar
  topics") and offered only SUPERSEDED / EVOLVED / CONTRADICTS /
  COMPATIBLE. But candidacy is exhaustive same-project: the cosine floor
  (CONTRADICT_MIN_SIM=0.60) is inert on multilingual-e5-large, where a
  measured 45% of all active pairs clear 0.78, so the judge is routinely
  handed same-project pairs about entirely different subjects. With no
  way to say "these are unrelated", the model scope-conflated shared
  vocabulary into false CONTRADICTS (observed: a hardened-VPS firewall
  memory flagged as contradicting an on-prem office LAN memory). The
  prompt now decides same-subject first and can answer UNRELATED;
  unrelated pairs are tombstoned as contradiction-cleared so they never
  re-enter the finder/judge loop. Validated by A/B on real
  same-project-different-subject pairs plus unambiguous contradiction
  controls: UNRELATED cleanly separates different subjects with zero
  regression on true contradictions (RAM 64 vs 32, nginx vs Apache,
  blood type, backup time all still flagged). Also replaced the ambiguous
  "weekly vs daily backup" CONTRADICTS example (those coexist) with a
  same-job time clash.