feat(hamplard): add instructor registry and token decimal validation

[Shredder401k]

Feature Enhancements and Security Fixes

Closes #148
Closes #149
Closes #151

Issue #148: Global instructor registry for efficient instructor-level operations

Problem: No global tracking of instructor addresses prevented efficient on-chain
instructor-level operations like freeze queries, earnings aggregation, and instructor listing.

Changes:

• Added InstructorRegistry key to DataKey enum to maintain global set of instructor addresses
• Updated register_course() to automatically add instructors to registry on first course registration
• Implemented get_all_instructors() admin function returning complete list of registered instructors
• Implemented get_instructor_count() admin function for platform analytics
• Registry is append-only and persists across course status changes
• Functions are admin-only to prevent privacy concerns

Code additions:

// In DataKey enum
InstructorRegistry,

// In register_course()
let mut registry: Vec<Address> = env
    .storage()
    .persistent()
    .get(&DataKey::InstructorRegistry)
    .unwrap_or_else(|| Vec::new(&env));

let mut exists = false;
for i in 0..registry.len() {
    if registry.get(i).unwrap() == instructor {
        exists = true;
        break;
    }
}

if !exists {
    registry.push_back(instructor.clone());
    env.storage()
        .persistent()
        .set(&DataKey::InstructorRegistry, &registry);
}

// New functions
pub fn get_all_instructors(env: Env, admin: Address) -> Vec<Address>
pub fn get_instructor_count(env: Env, admin: Address) -> u32

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