docs: add ARCHITECTURE.md (#21)

[malaysiaonelove]

Summary

Adds ARCHITECTURE.md — the canonical architecture reference for the Orivex Contracts repository, as specified in issue #21.

Closes #21

What's included

• Section 1 — System Overview: Mermaid diagram of all 6 contracts, callers, and USDC SAC token
• Section 2 — Contract Roles: address aliases and one-line responsibilities
• Section 3 — Cross-Contract Call Graph: sequence diagram + call table with source file line links
• Section 4 — Trust Boundaries: principal hierarchy, 7 trust invariants, what each contract does NOT trust, recovery flows
• Section 5 — Per-Contract Deep-Dives: storage layout tables, auth model, events, key constants for all 6 contracts
• Section 6 — Wiring Guide: ordered deployment + initialization steps with exact stellar contract invoke commands and a Mermaid dependency flowchart
• Section 7 — Panic Catalog: every panic message, its trigger condition, and source function for all 6 contracts
• Section 8 — Extension Guide: adding a new contract, event, admin role, storage key; generating API docs via cargo doc

Also adds .markdownlint.json to configure lint rules repo-wide (disables MD013 line-length, scopes MD024 to siblings-only).

Acceptance criteria checklist

• Document is lint-clean (markdownlint-cli2: 0 issues)
• Renders correctly on GitHub (Mermaid 10.x, standard GFM tables)
• Cross-references source files for every claim (line-linked in call table and per-contract sections)