Implement 'Project Whitelist' for Institutional Investors #298

Context

Large projects might want only verified hedge funds.

Problem

Access control is currently all-or-nothing.

Suggested Execution

• Add 'investor_whitelist' mapping to project.
• Admin adds addresses. 'contribute()' checks whitelist.

Acceptance Criteria

• Only whitelisted addresses can invest.
• Whitelist management is simple for admins.

References

• contracts/project-launch/src/whitelist.rs