fix: round refund amounts correctly instead of flooring

[daameeloolaa-png]

fix: round refund amounts correctly instead of flooring

Problem

Three separate integer division bugs caused small token amounts to be silently discarded
rather than returned to users during refunds and cancellations.

Changes

contracts/nft-stream/src/lib.rs — calculate_vested

The vested amount was computed as (total_amount * elapsed) / duration, a single
multiply-then-divide. Integer floor division means vested is always rounded down, so
refund_amount = total_amount - vested is inflated — the sender recovers more than owed while
the recipient loses up to duration - 1 base units on cancellation. The intermediate product
also risks overflow for large amounts.

Replaced with the split-multiplication identity already used in payment-stream:

(total / duration) * elapsed + ((total % duration) * elapsed) / duration

contracts/campaign-funding/src/lib.rs — calculate_reserve

The 10% tree-replacement reserve used floor division on the remainder term, so it could come
out 1 stroop short. That dust ended up in the distributable amount instead of the reserve,
silently under-funding it.

Applied ceiling division (+ 9_999 before dividing) to match the existing pattern in
calculate_fee.

contracts/campaign-funding/src/lib.rs — distribute_proceeds

Each team member's share was computed with floor division on the remainder. With multiple
members the per-member dust accumulates and stays locked in the contract permanently.

Earlier members now use ceiling division; the last member receives amount - distributed (the
exact leftover), guaranteeing sum(all shares) == amount with nothing stranded.

Impact

• No change to the happy path for whole-number amounts.
• Maximum correction per operation: 1 base unit (1 stroop for XLM, 1 unit for USDC).
• All three fixes are backwards-compatible — no storage layout changes.

closes #734

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