Loan manager liquidate

[EbukaMoses]

close #15

Acceptance criteria

[x] Adjust total_outstanding on liquidation to remove the liquidated loan's outstanding principal, consistent with repay/check_default
[x] Verify available-liquidity accounting in approve_loan after a liquidation
[x] Add a test that liquidates a loan and asserts subsequent approve_loan sees the freed liquidity
[x] Confirm no path leaves TotalOutstanding permanently inflated

[ogazboiz]

the liquidation change and the new test look reasonable, but the pr needs cleanup before it can be reviewed properly:

1. it commits the entire target/ build directory (around 96 files: fingerprints, .cargo-lock, build scripts) plus a 2,665-line test snapshot json. please drop the build artifacts: add target/ to .gitignore and run git rm -r --cached target && git commit, and reconsider whether the test_snapshots json belongs in the pr. right now the real change (loan_manager/src/lib.rs +8/-5 and test.rs) is buried under build output.

2. ci fails on cargo fmt: loan_manager/src/test.rs:1537 has assert_eq! calls that fmt wants collapsed onto one line. run cargo fmt from the contracts root and commit.

3. the branch is named LoanManager_liquidate, please use something like feat/loan-manager-liquidate to stay consistent with the other branches:

   git branch -m LoanManager_liquidate feat/loan-manager-liquidate
   git push origin -u feat/loan-manager-liquidate
   

once target/ is out of the diff, fmt is clean and ci is green, i'll review the liquidate logic in detail.

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