Add per-project investor whitelist to project-launch contract

[Chrisland58]

Add per-project investor whitelist

Closes: Access control is all-or-nothing — large projects (e.g. hedge fund-only rounds) had no
way to restrict who could contribute.

Changes

• whitelist.rs (new) — per-project whitelist module with enable/disable toggle, add/remove
  investor, and is_blocked() gate
• lib.rs — wires is_blocked() into contribute() before the KYC check; exposes 5 new contract
  entry points; adds 6 tests
• shared/errors.rs — adds NotWhitelisted = 51, WhitelistEnabled = 52
• shared/events.rs — adds WHITELIST_ENABLED, WHITELIST_DISABLED, INVESTOR_ADDED,
  INVESTOR_REMOVED

Behaviour

• Whitelist is opt-in per project — disabled by default, so existing projects are unaffected
• Only the project creator can manage the whitelist (no platform admin involvement)
• When disabled, is_blocked() short-circuits with zero overhead

Testing

• Non-whitelisted investor blocked when whitelist enabled
• Whitelisted investor can contribute
• Removing an investor revokes access immediately
• Disabling whitelist re-opens contributions to all
• Non-creator cannot manage whitelist
  closes Implement 'Project Whitelist' for Institutional Investors #298

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