docs: Add JS architecture, testing strategy, API examples, and security hardening

[famvilianity-eng]

Summary

Comprehensive documentation updates across four areas:

• Add architecture diagram covering the JS batch/insurance/matching/reputation/royalty layer #899 — Add JS processing layer (batch, insurance, matching, reputation, royalty) architecture diagram showing integration with Soroban contracts and REST API
• Add a docs/testing-strategy.md documenting the disabled-module cleanup plan #900 — Create docs/testing-strategy.md with full test suite status, tracking cleanup plan tied to issue Add SECURITY.md with vulnerability disclosure policy #82
• Update docs/integration-guide.md with real request/response examples once handlers are wired #901 — Add real REST API request/response examples for all core operations (list IPs, get IP, initiate/accept swap, reveal key)
• Update docs/security.md with request-signing and RPC circuit-breaker details #902 — Document Ed25519 request-signing scheme, rate-limit tiers, circuit-breaker state machine, and Prometheus metrics

Changes

Architecture (#899)

• Added "JavaScript Processing Layer" section showing batch, insurance, matching, reputation, royalty modules
• Mermaid diagram of JS layer integration with Soroban contracts
• Module responsibilities table
• Integration flow explanation

Testing Strategy (New file, #900)

• Complete inventory of enabled test modules across ip_registry and atomic_swap
• List of 5 disabled test modules with reasons and cleanup tracking
• Cleanup plan with phased re-enabling steps
• Test execution commands and re-enable instructions
• Known issues section

Integration Guide (#901)

• Added REST API section with real JSON request/response examples
• Covers: list IPs, get IP record, initiate swap, accept swap, reveal key
• Includes success (200/201) and error (400/404) responses
• Realistic data shapes and values

Security Hardening (#902)

• Request Signing: Ed25519 signature scheme, signing payload format, header requirements, example Python code
• Rate Limiting: Tier table (Free/Premium/Enterprise), token-bucket algorithm, rate-limit headers, backoff strategy, multi-instance deployment modes
• Circuit Breaker: State machine diagram, configuration parameters, named breakers, metrics, Prometheus alerting expressions

Closes #899
Closes #900
Closes #901
Closes #902

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