feat: implement batch_transfer, batch_mint, batch_approve (#160)

[paulinaapeh8]

• Add MAX_BATCH_SIZE=50 constant with BatchTooLarge/BatchEmpty errors
• batch_mint: validate all entries fail-fast, emit batch event, atomic
• batch_transfer: pre-validate total, handle self-transfer edge case
• batch_approve: approve multiple spenders atomically, zero revokes
• Add emit_batch_transfer, emit_batch_mint, emit_batch_approve events
• Add 24 comprehensive tests covering empty, single, multi, max size, too large, invalid amounts, paused state, overflow, self-transfer, zero-amount revoke
• SDK: add batchTransfer(), batchApprove(), BatchTransferRecipient, BatchApproveSpender types; export from index.ts
• Fix pre-existing compile errors: FeeExemption u8->u32, duplicate private helper names, ledger().set() API, missing Ledger trait imports

Summary

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to change)
• 📝 Documentation update
• 🔧 Smart contract improvement
• 🧪 Test coverage improvement
• 🏗️ CI/Build improvement

Related Issue

Closes #

Changes Made

Testing

How has this been tested?

• Rust unit tests pass (cargo test)
• SDK compiles (npm run build in sdk/)
• Manual testing against Soroban testnet
• New tests added for changes

Test commands run:

# Contract tests
cargo test

# SDK build
cd sdk && npm run build

Checklist

• My code follows the project's style guidelines
• I have added NatSpec-style comments to new Rust functions
• I have added JSDoc comments to new TypeScript functions
• I have updated the README if needed
• My branch follows the naming convention: feature/<issue-number>-<description>
• I have not modified files outside the scope of this issue

Drips.network Contributor Info

• Drips Profile:
• Issue Claimed: #

Screenshots (if applicable)

Closes #160

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