Configurable rounding policy for reward math

[joelpeace48-cell]

Epic: Advanced Contract Features

Summary

Expose whether bps/multiplier math rounds down, up, or to nearest, per campaign.

Background & Problem

Different programs need different rounding; making it explicit prevents silent value drift and disputes.

Acceptance Criteria

• Rounding mode configurable and documented.
• Math is consistent + tested for each mode.

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Filed as part of the mainnet-readiness & world-class roadmap. Scope is intentionally small enough for one focused PR; comment to claim.