[Testing] calculate_claimable underflow guard (withdrawn > deposited -> 0) is never exercised

[grantfox-oss]

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Why this matters

calculate_claimable uses checked_sub(...).unwrap_or_default() at lib.rs:347-350 to defend against withdrawn_amount > deposited_amount. The fuzz test that calls calculate_claimable directly only generates withdrawn = seed % (deposited+1), so withdrawn never exceeds deposited and the underflow-to-0 fallback is dead in tests.

Acceptance criteria

• Construct a Stream with withdrawn_amount > deposited_amount and call calculate_claimable, asserting it returns 0 (no panic/underflow)

Files to touch

• contracts/stream_contract/src/test.rs
• contracts/stream_contract/src/lib.rs

Out of scope

• The rate*elapsed overflow cap (already covered)

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