refactor: drop shadowed-import + duplicate-definition cruft (-91 LOC)

Left over from the data.py/vhack.py/eval.py/tablelog.py module split. In
train.py the canonical imports already won at runtime; the earlier ones were
dead shadows:
- ablate_quarantine, ref_logprobs_via_zero_delta: .eval wins (line 66), drop
  the .antipasto copy; load_v_hack/postprocess_v_hack: .vhack wins, drop
  .extract_vhack_grad; DATA/load_problems: .data wins, drop .problems.
- local setup_logging() was byte-identical to the .tablelog one already
  imported (with StepLogger); delete the local def + now-orphaned datetime
  import and LOGS_DIR const.
- problems.py stays: 6 scripts + derisk/regrade still import it.

antipasto.py: delete detach_antipasto (0 callers) and its own copies of
ref_logprobs_via_zero_delta / ablate_quarantine (eval.py owns the canonical,
better-worded versions incl. the SGTM TODO), plus now-unused contextmanager
and per_token_logps imports.

docs: rm corrupted docs/spec/20260530_substrate_review_qwen.md (2-line API
error dump, not a review).

Behavior-preserving (later imports already won at runtime). Verified: just
smoke (erase) + just smoke-routeV both exit 0, 0 tracebacks, all verify_*
gates PASS.

Co-Authored-By: Claudypoo <288921227+claudypoo@users.noreply.github.com>

docs/spec/20260530_substrate_review_qwen.md

@@ -1,3 +0,0 @@
-400 Provider returned error
-{"error":{"message":"developer is not one of ['system', 'assistant', 'user', 'tool', 'function'] - 'messages.['0].role'","type":"invalid_request_error","param":null,"code":null},"request_id":"chatcmpl-8036d119-9aa0-981f-9113-99a865d3f90e"}
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