feat: add end-to-end mock flow fixtures (#184)

[Justine-Ifedozie]

Closes #184

Summary

Adds end-to-end mock flow fixtures that chain the existing per-domain
fixtures (compliance, asset issuance, minting, portfolio, transactions,
diagnostics) into three coherent investor journeys, without duplicating
any fixture data.

What's included

• src/fixtures/flows.ts — 3 named scenarios covering all three reviewable
  compliance outcomes (compliant / restricted / pending_review)
• src/__tests__/fixtures/flows.test.ts — 14 tests verifying referential
  integrity against the underlying fixtures
• docs/mock-flow-fixtures.md — usage docs and how to add new scenarios
• docs/mock-mode.md — updated fixture table to link the new doc

Acceptance criteria

• Mock flow fixtures are added
• Compliance and asset scenarios are included
• Minting and transaction scenarios are included
• Investor portfolio scenarios are included
• Docs explain mock usage
• Mock mode is clearly distinguished from live data (unchanged — still
  gated by NEXT_PUBLIC_MOCK_MODE / assertMockModeSafe())

Testing

npm run lint, npm run typecheck, npm test (561/561 passing), and
npm run build all verified clean locally before pushing.