Implement zero amount and dust handling

[TUPM96]

Resolves #380

Summary:

• add shared dust checks and reject zero/negative configuration values before state changes
• prevent withdrawals and repayments from leaving residual deposit/debt dust
• add user sweep functions for existing deposit and debt dust balances
• use depositor-friendly ceiling interest rounding while keeping I256 intermediate math
• expose the existing stable-rate contract methods needed by the lending tests and seed stable-rate state at initialization
• document zero/dust semantics and add edge-case tests

Verification:

• cargo fmt -p stellarlend-lending -- --check
• cargo check -p stellarlend-lending
• cargo test -p stellarlend-lending -- --test-threads=4
• git diff --check

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