PR_BODY_ISSUE_796.txt

## Summary

This PR implements batch query functionality for claim verification across multiple subjects in a single contract call, addressing performance concerns for bulk operations like airdrops, marketplace eligibility checks, and cross-contract verification.

## Problem Statement

Previously, verifying claim validity for multiple subjects required individual `has_valid_claim` calls for each address. For use cases like:
- Airdrop allowlist verification (checking 100+ addresses)
- Bulk permission checks before operations
- Cross-contract verification loops
- Marketplace user eligibility batching

This resulted in N contract invocations, which is expensive at scale and creates unnecessary latency.

## Solution

Introduces `has_valid_claim_batch()` function that:
- Accepts a vector of addresses and a single claim type
- Returns a vector of boolean values (one per subject)
- Executes in a single contract call
- Uses existing `has_valid_claim` logic internally
- Maintains OR-logic across issuers for each subject

## Changes

### Implementation Details

**src/query.rs**:
```rust
pub fn has_valid_claim_batch(env: &Env, subjects: Vec<Address>, claim_type: String) -> Vec<bool> {
    let mut results = Vec::new(env);
    for subject in subjects.iter() {
        let has_claim = has_valid_claim(env, subject.clone(), claim_type.clone());
        results.push_back(has_claim);
    }
    results
}
```

**src/lib.rs**:
```rust
#[must_use]
pub fn has_valid_claim_batch(env: Env, subjects: Vec<Address>, claim_type: String) -> Vec<bool> {
    query::has_valid_claim_batch(&env, subjects, claim_type)
}
```

### Usage Example

```rust
// Before: Multiple expensive calls
let kyc1 = contract.has_valid_claim(&subject1, "KYC_PASSED");
let kyc2 = contract.has_valid_claim(&subject2, "KYC_PASSED");
let kyc3 = contract.has_valid_claim(&subject3, "KYC_PASSED");

// After: Single efficient call
let results = contract.has_valid_claim_batch(
    vec![subject1, subject2, subject3],
    "KYC_PASSED"
);
// results[0] = kyc1, results[1] = kyc2, results[2] = kyc3
```

## Technical Details

- **OR-logic Preservation**: Each subject's claim validity maintains multi-issuer OR-logic semantics
- **Storage Access**: Leverages existing storage patterns (no new storage keys)
- **Error Handling**: No validation errors; returns bool array (consistent with single-subject version)
- **Backwards Compatible**: Fully backwards compatible, no breaking changes
- **Performance**: Reduces network calls from N to 1 for bulk operations

## Testing

Verified:
- ✅ Library compiles without errors
- ✅ Function signature correct
- ✅ No new dependencies
- ✅ Follows project conventions
- ✅ Cargo build --lib passes

## Files Modified

- `src/query.rs`: +14 lines (batch query logic)
- `src/lib.rs`: +6 lines (public function wrapper)

## Closes

#796