docs: add 5-phase med-mal records platform roadmap

Closes the documentation gap surfaced after Phase 1 shipped: the
per-phase plans (PLAN_med_mal_templates.md, PLAN_med_mal_extraction_pipeline.md)
were already committed, but the meta-orchestration that sequences them
lived only in a local Claude planning file. Future contributors (and
resumed sessions) now see the whole arc in one repo-resident document.

The roadmap captures:
- Why Phase 0 (research-validation pass) precedes Phase 2 - cheap
  insurance that produced 10 deltas to the extraction-pipeline plan
  including a missing causation rule, missing § 145.64 peer-review
  hard-refuse, and missing Plutshack/Smith 4-cut tagging.
- Why Phase 1 (templates) precedes Phase 2 (extraction) - exercises the
  Supabase-branch + schema-migration toolchain on a 1-column reversible
  change before the 3-table change.
- Four hidden dependencies surfaced from codebase exploration that the
  per-phase plans don't address: missing backend/migrations/ directory,
  text-only LLM provider abstraction, no bbox-extraction primitive,
  free-plan Supabase branching limit.
- Phase status (0 + 1 shipped at 3a97315 and 73faac0; 2/3/4 pending).
- Concrete Phase 2 first steps + the multimodal/bbox sub-tasks hidden
  inside the multi-day estimate.
- Phase 3 architectural decision (event log stays narrow; per-schema
  extractors consult it rather than expanding the log).
- Phase 4 v1.1 deadline-tracking surface tied to Phase 2's
  document_events.key_date_role column.
- Cross-phase risk register, out-of-scope list with permanent
  architectural separations (causation-chain reasoning,
  provider-defendant entity resolution).

PDF rendered via reconstructed /tmp/jenn-mike/md_to_pdf.py.