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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[Miracle656]

The type-level tests (test-d/index.test-d.ts), credential-format and xdr-roundtrip tests are valuable additions. Two things before this can merge:

1. Conflicts — this branch is behind main and now conflicts. Please rebase on the latest main (the sdk package-lock.json and tsconfig.json have moved).
2. Scope — the PR also modifies frontend/website/app/page.tsx and components/WhyVeil.tsx, which seem unrelated to "type-level tests and test coverage." Could you split those out (or explain the connection)? Keeping the PR scoped to the SDK tests will make it much faster to review and merge.

Once rebased and scoped to the test work, I'll merge it.

[thegreatfeez]

Superseded by #297. Rebased on latest main and scoped to SDK tests only — frontend changes (WhyVeil.tsx, page.tsx) removed as requested.