Explicit zero-amount and zero-address input guards across entry points

[joelpeace48-cell]

Epic: Contract Safety & Hardening

Summary

Reject zero amounts and zero/invalid addresses early with typed errors.

Background & Problem

Missing zero-checks cause wasted fees, confusing no-ops, and can mask bugs.

Acceptance Criteria

• All entry points reject invalid zero inputs.
• Typed errors returned.
• Covered by tests.

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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.

[JuanVP2]

I'd like to work on this issue. I will implement early validation checks across all entry points to reject zero amounts and invalid addresses using typed errors, and include comprehensive test coverage.