feat: add type-level tests and improve test coverage

[thegreatfeez]

Summary

This PR introduces robust multi-layered testing infrastructure for the core SDK to protect against architectural regressions, while updating the marketing landing page to clearly articulate Veil's unique value proposition.

Related issue

Closes #257
Closes #260
Closes #261
Closes #262

Type of change

• Bug fix
• New feature
• Refactor
• Docs
• Tests
• CI / tooling

Component

• Wallet frontend
• SDK
• Contracts
• Agent

Checklist

• I have read CONTRIBUTING.md
• cargo test passes (contracts)
• npm run typecheck passes (wallet / sdk / agent)
• npm run build passes (wallet / agent)
• I added or updated tests where relevant
• I updated docs / README where relevant

---

Description

This pull request implements stability enhancements, testing infrastructure, and critical positioning updates for both the SDK and the wallet frontend marketing components.

Key Deliverables

1. SDK Core Architecture & Stability

• XDR Encoding Roundtrip Tests: Added sdk/tests/xdr-roundtrip.test.ts to execute end-to-end encode → decode → assert deep equality checks across all transaction builder helpers to eliminate subtle serialization mutations.
• Type-Level Regression Suite: Established compile-time type assertions using tsd in sdk/test-d/index.test-d.ts (10+ type assertions tracking high-traffic SDK return types). Integrated npm run test-d into the pipeline via sdk/package.json.
• State Format Pinning: Added a backward-compatibility snapshot test (sdk/tests/credential-format.test.ts) using a v1 fixture configuration (sdk/tests/fixtures/credential-v1.json) to prevent storage deserialization breaks for existing wallets. Codified and documented the project's long-term format-stability policy.

2. Frontend Marketing Positioning

• Value Proposition Component: Designed and integrated frontend/website/components/WhyVeil.tsx into the main application landing page (frontend/website/app/page.tsx).
• Competitive Contrast Matrix: Built a mobile-responsive comparison table contrasting Veil against legacy alternatives (Freighter and xBull), emphasizing its core architecture pillars:
  • No seed phrases
  • No browser extension requirements
  • Native Account Abstraction
• Performance Metric Compliance: Validated layout structure to guarantee a Lighthouse accessibility score of $\ge 95$.

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