docs: add MN med-mal law research and update templates plan

Three new docs and a substantive revision to the templates plan, all
captured in `docs/`:

- `RESEARCH_mn_med_mal_law.md/.pdf` - validates the med-mal-case template
  against Minnesota primary sources (statutes, MN Supreme Court opinions,
  Rules of Civil Procedure). Corrects two inherited errors: § 145.682(4)'s
  180-day clock runs from Rule 26.04(a) discovery commencement (not
  summons service), and the MN prima facie negligence test is the
  3-element Plutshack/Smith v. Knowles formulation (not the 5-element
  informed-consent test). Ends with a 10-item "Plan deltas" appendix.
- `RESEARCH_legal_rag.md` - surveys legal RAG approaches across case-law,
  contracts, e-discovery, and med-mal lanes; recommends structured
  extraction with page+bbox citations over vector RAG for med-mal records.
- `PLAN_med_mal_extraction_pipeline.md` - Mike-specific build plan derived
  from the RAG research: page-by-page multimodal extraction → JSON event
  log in Postgres → deterministic red-flag rules → page-window retrieval
  for chat. Out-of-scope: embeddings, GraphRAG, cross-case search.
- `PLAN_med_mal_templates.md` - applies all ten plan deltas: subfolder
  taxonomy expanded 8→12+nested (adds expert-affidavits-145682,
  hipaa-authorizations, discovery, collections-liens-subrogation,
  pre-retainer-investigation; renames bills→bills-and-eobs); three new
  tabular schemas (provider-defendant-map, causation-chain,
  expert-opinions-145682); bills columns split per § 548.251; red-flag
  scan gains § 145.682 affidavit link + temporal-proximity anchor; new
  "Cited authority" section consolidates statutes/rules/cases for future
  revision audits. v1 schema change stays at single `template_id` column;
  v1.1 follow-up note flags `key_dates jsonb null` for deadline tracking.