docs: add mobile vault SDK integration risk document and feature guide

[kaizercodes]

Overview

Adds a comprehensive mobile vault SDK integration risk document and feature module architecture guide, detailing ecosystem boundaries, dual-mode execution (mock vs live Soroban contract), SDK/contract assumptions, error recovery models, and cross-repository handoff points.

Related Issue

Closes #205

Changes

Documentation & Specifications

• [MODIFY] docs/vault-integration-risks.md
  • Expanded into an exhaustive technical risk analysis covering dual-mode execution, SDK helper interfaces, BigInt/i128 stroop precision, Soroban contract entrypoints, ledger consensus time vs client device clocks, error taxonomy, placeholder behavior matrix, and technical debt.
• [MODIFY] docs/vault-integration-assumptions.md
  • Added cross-reference links to the vault feature module and updated related documentation index.
• [MODIFY] README.md
  • Updated documentation and feature summaries with references to the vault integration risk and feature module documentation.

Feature Module Documentation

• [ADD] src/features/vault/README.md
  • Added architecture documentation for the vault feature module, detailing components (DepositPreview, WithdrawalPreview), hooks (useVaultDepositForm, useMaturedLockWithdrawal), stores (vaultStore.ts), error helpers, and dual-mode execution paths.

Verification Results

$ npm run api:check
pocketpay-sdk public API matches baseline (2 exported member(s)).

$ npm test -- __tests__/vaultStore.withdrawMaturedLock.test.ts
PASS __tests__/vaultStore.withdrawMaturedLock.test.ts
  vaultStore.withdrawMaturedLock (9 passed, 9 total)

$ npm test -- __tests__/VaultUnavailableState.test.tsx
PASS __tests__/VaultUnavailableState.test.tsx (5 passed, 5 total)

$ npm test -- src/utils/__tests__/vaultErrors.test.ts
PASS src/utils/__tests__/vaultErrors.test.ts (35 passed, 35 total)

$ npm test -- src/utils/__tests__/vaultAvailability.test.ts
PASS src/utils/__tests__/vaultAvailability.test.ts (9 passed, 9 total)

Acceptance Criteria	Status
A vault integration risk document is added / expanded	✅ docs/vault-integration-risks.md comprehensively documents all integration risks and operational models
SDK assumptions are listed	✅ Interface signatures, Stroops BigInt conversions, key custody boundaries, and capability gates documented
Contract assumptions are listed	✅ Soroban entrypoints (deposit, withdraw, balance, get_locks, unlock), ledger consensus time, and SAC token custody documented
Placeholder UI behaviour is clearly identified	✅ Comparative matrix across all UI touchpoints differentiating mock mode from production target behavior
Known integration risks are documented	✅ Detailed technical debt and risk matrix covering clock skew, schema drift, RPC reliability, and fee volatility with mitigations