docs(architecture): add diagrams and implementation playbook

Added three new documents with nine Mermaid diagrams to make the strangler
fig strategy visible:

- README: container topology diagram at the start, with the proxy entry point
  and three seams labelled
- docs/architecture.md: five diagrams tracing the exact implementation:
  - The four seams and who holds authority at each boundary
  - How by-id read goes through the resolver, but list-read bypasses it
  - Case lifecycle state machine (the strategy in one picture)
  - Take-ownership sequence with failure windows annotated
  - Write-through error round-trip showing zero validation logic crossed
- docs/playbook.md: how to apply this to a production system:
  - Write-path decision tree (five read/write patterns)
  - Cutover ordering diagram (side-effects-free first, least recoverable last)
  - Seven transferable rules with pointers to the files that demonstrate them
  - Scope diagram of what's proven vs. left as your decisions

Resolved all 13 dangling § citations (to an absent spec doc) by linking to
the actual files or dropping them. Replaced portal-frontend/README.md
boilerplate with accurate content. All diagrams parse and link-check clean.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

docs/adr/ADR-003-ownership-is-taken-per-case.md

@@ -22,7 +22,7 @@ their keep.
 
 ## Decision
 Ship now with ownership taken **one legacy case at a time**, via
-`POST /api/worklist/legacy/{aanvraagId}/take-ownership` (§7.5), triggered by
+`POST /api/worklist/legacy/{aanvraagId}/take-ownership`, triggered by
 an explicit user action in the portal. This is the interim mechanism, not the
 final one for every process.
 
@@ -43,11 +43,12 @@ This is deliberately the right building block either way:
 - Until bulk tooling exists, full legacy retirement for a process happens
   case-by-case, which is slower than a scheduled cutover — accepted as the
   cost of shipping the seam mechanics now rather than waiting.
-- Reversal (§7.6) stays per-case and gated on `domain_writes_since` for the
+- Reversal (`ReleaseOwnershipHandler`) stays per-case and gated on `domain_writes_since` for the
   same reason a bulk reversal would be unsafe absent a sync
   (`docs/sync-not-implemented.md`): undoing adoption after edits would
   silently discard them.
-- This demo's non-goals (§3) exclude building the bulk migration tool itself
+- This demo's non-goals (README, "Deliberate substitutions and omissions")
+  exclude building the bulk migration tool itself
   — that's future work, not a rejected idea.
 
 ## Future work