@ feat(lab): SECT-INDEX carries body-relative @N offsets; find shares the markdown plane

- structureIndexExperiment: index lines anchor into the served markdown and
  emit @N body offsets (line-anchored display for article/section/schedule,
  parent-relative enum token for subsections). Verified on the covenants docx:
  309/310 spine lines anchored, 0 bad slices.
- localAssistantTools: servedDraftingText shares one served-text computation
  between read_document and find_in_document (find-plane change gated on the
  index flag only, so e2e/markdown-swap arms keep the plaintext find surface);
  offset/max_chars windows are body-relative under the index arm; scoped-only
  refusals harmonized to head:20 + @N.
- upstreamMikeBenchmarkSurface: orient-first prompt is now batching-aware and
  points at the @N offsets.
- deepseek: snapshot the tool list once per tool-loop so the serialized prefix
  stays byte-stable across rounds (prefix-cache identity).

Co-Authored-By: Claude <noreply@anthropic.com>