fix: resolve issues #138, #137, #136, #25

[thelux134]

#138 - Treasury reentrancy guard:

• Remove local ReentrancyGuard struct; use kora_shared::reentrancy::ReentrancyGuard
• Fix emergency_withdraw double-transfer bug (was calling transfer twice)
• Replace manual acquire_lock/release_lock with RAII guard in both withdraw and emergency_withdraw
• Add missing WhitelistedToken variant to DataKey enum

#137 - Standardize Risk Registry events:

• Add registry_initialized event to shared events module
• Emit registry_initialized from RiskRegistryContract::initialize()
• All other operations already used shared events consistently

#136 - Shared Contracts Library:

• Remove duplicate ReentrancyGuard struct definition in reentrancy.rs
• Single canonical RAII guard with acquire_guard/release_guard/is_locked helpers

#25 - Invoice NFT documentation:

• Add MigrationVersion variant to DataKey enum (was used in code but undeclared)
• Update DataKey doc comment to list all variants including MigrationVersion
• Remove stale InvoiceCount reference from storage key comment block
  Closes Fix Reentrancy Guard in Treasury Contracts #138
  closes Standardize Events in Risk Registry Contracts #137
  closes Implement Shared Contracts Library #136
  closes Update Documentation for Invoice NFT Contracts #25

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