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-001-decision-independent-of-closure.md

@@ -7,7 +7,8 @@ Accepted.
 `case-framework` (seam D, a stand-in for a maintained vendor case-management
 framework) refuses `POST /cases/{id}/closure-request` with **409 Conflict**
 while any task on the case is still open. That rule belongs to the framework
-and is not ours to change — it is a conformist integration by design (§6).
+and is not ours to change — it is a conformist integration by design
+(seam D, `New.Infrastructure.CaseFramework/CaseFrameworkGateway.cs`).
 
 The new domain's own rule is different: once an assessment (approve/reject) is
 recorded on a `RegistrationApplication`, that decision is legally in effect