fix: move BorrowerLoans from instance to persistent storage

[Fury03]

Closes #1086

Problem Statement (The Bug)

The per-borrower list of loan ids (BorrowerLoans(Address)) is stored in instance storage, which is loaded in full on every contract call. Since the list only ever grows, every call gets more expensive until the contract can brick. This is per-borrower unbounded data that belongs in persistent storage keyed per borrower.

Instance storage is one bucket loaded in full on every invocation. An unbounded, append-only list per borrower inflates that entry over time, raising read/write cost on every call and eventually pushing toward the instance size limit, at which point the whole contract becomes unusable. This cannot be fixed with a local patch because it's a fundamental storage layout issue — instance storage simply cannot hold unbounded per-key data.

Solution Comparison and Decision

Option A: Paginate the instance-stored list

• Still grows in instance storage, just slower
• Adds client complexity for pagination
• Doesn't solve the root cause

Option B: Move to persistent storage keyed per borrower

• Persistent storage is keyed and only the specific entry is loaded
• Unbounded growth doesn't affect other storage entries
• Consistent with how BorrowerLoanCount and individual Loan records are already stored
• TTL bump applies cleanly

Option C: Store in a Soroban contract with Map

• Over-engineered for a simple append-only list
• Adds cross-contract call overhead

Chosen: Option B — Moving BorrowerLoans(Address) from instance to persistent storage is the only correct path. It's how Soroban is designed to handle per-key unbounded data.

The Change (Code modifications)

Entry point	Before	After
request_loan (write)	instance().get/set(BorrowerLoans)	persistent().get/set(BorrowerLoans) + TTL bump
get_borrower_loans (read)	instance().get(BorrowerLoans)	persistent().get(BorrowerLoans) + TTL bump

// Before (instance — loads entire instance on every call):
let mut borrower_loans: Vec<u32> = env
    .storage()
    .instance()
    .get(&borrower_loans_key)
    .unwrap_or(Vec::new(&env));
borrower_loans.push_back(loan_counter);
env.storage()
    .instance()
    .set(&borrower_loans_key, &borrower_loans);

// After (persistent — only loads this specific entry):
let mut borrower_loans: Vec<u32> = env
    .storage()
    .persistent()
    .get(&borrower_loans_key)
    .unwrap_or(Vec::new(&env));
borrower_loans.push_back(loan_counter);
env.storage()
    .persistent()
    .set(&borrower_loans_key, &borrower_loans);
Self::bump_persistent_ttl(&env, &borrower_loans_key);

get_borrower_loans similarly reads from persistent() and bumps TTL.

Migration documentation added in the migrate() function explaining that existing instance-stored lists cannot be automatically migrated (Soroban doesn't support iterating instance keys), but this is safe because:

1. BorrowerLoanCount (used for cap enforcement) was already in persistent storage
2. Individual Loan records were already in persistent storage
3. New loans will be tracked correctly in persistent storage

Compatibility Note

INTERFACE_VERSION is not modified (not applicable to this contract's interface pattern). The CURRENT_VERSION constant is bumped from 4 to 5 to mark the storage layout change. The external API (get_borrower_loans return shape) is unchanged.

Incidental Fixes

• Added TTL bump on persistent BorrowerLoans entries (previously missing even if they were in persistent storage)
• Migration guard updated to handle the new version

Testing

All 130 tests pass:

test result: ok. 130 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

Key test: test_get_borrower_loans — verifies write-then-read round trip, which now exercises persistent storage. The test creates two loans for a borrower, verifies both are returned, repays one, and confirms the list still contains both (historical record).

cargo fmt --check — clean
cargo clippy — only pre-existing doc warning in events.rs (unrelated)

Additional Notes

Test snapshots were regenerated (gitignored) due to the version bump. No changes to shared code — only contracts/loan_manager/src/lib.rs and contracts/loan_manager/src/test.rs were modified.