Add contract invariants for zero or negative readings

[AnnabelJoe]

Category: Backlog

Description: Add contract invariants for zero or negative readings.

Note: recreated without labels after earlier label errors.

[teethaking]

@teethaking has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

hello maintainer may i work on this issue?
ill submit PR within 1-3 days

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[omonxooo-commits]

@omonxooo-commits has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

love to work on this

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[Vera3289]

@Vera3289 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

please assign boss, I can work on it

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[drips-wave]

Congratulations, @Vera3289! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @Vera3289: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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[drips-wave]

This issue has been completed by @Vera3289 as part of the Stellar Wave Program's 6th Wave 🥳

😎 @Vera3289: You earned 200 Points for completing this issue! After the current Wave ends, you'll be eligible for a percentage of the Wave's reward pool based on the percentage of total points you've earned. Learn more here. You can also Leave a review to share your experience working on this issue.

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