docs(architecture): add top-level technical design doc

Closes round 4 of doc-prep. ARCHITECTURE.md is the day-1 read for
engineers joining Cyclone: process & network topology, tech stack
(backend + frontend + external), backend package layout with
one-line module responsibility map, frontend architecture (route
map, state management, live tail), data model (migrations 0001-0012,
ERD, 7-state claim lifecycle, audit chain), data flow (manual upload,
SFTP polling, auto-match, outbound 837P, ack ingestion), API surface,
cross-cutting concerns (logging, audit, PHI/PII, error model),
operational concerns (startup order, schedulers, health probe,
shutdown), deployment, out-of-scope, and references.

759 lines, 13 top-level sections. Cross-linked from REQUIREMENTS.md
sister-docs block and from README.md Roadmap section.

README.md

@@ -801,7 +801,11 @@ backup API).
 
 ## Roadmap
 
-> **Cross-cutting requirements, architecture, traceability, and definition of done live in [`docs/REQUIREMENTS.md`](docs/REQUIREMENTS.md).** Read that first if you are new — it is the single index that ties the 22 shipped sub-projects to the 38 functional requirements, the 18 non-functional requirements, the test strategy, and the project-level DoD.
+> **Read order for new engineers:**
+> 1. [`docs/REQUIREMENTS.md`](docs/REQUIREMENTS.md) — what Cyclone does (FRs + NFRs + DoD + traceability). The single index tying the 22 shipped sub-projects to the 38 functional + 18 non-functional requirements and the test strategy.
+> 2. [`docs/ARCHITECTURE.md`](docs/ARCHITECTURE.md) — how it fits together (process topology, package layout, data flow, lifecycle, operational concerns).
+> 3. The per-SP spec under [`docs/superpowers/specs/`](docs/superpowers/specs/) for whatever you're touching.
+> 4. The per-SP plan under [`docs/superpowers/plans/`](docs/superpowers/plans/) if you're implementing.
 
 Sub-projects 2 through 19 are **shipped**. See the [completeness
 review](docs/reviews/2026-06-20-cyclone-completeness-review.md) for