Feature: income_proof range mode — prove income within a band, not just above a threshold

[Psalmuel01]

income proofs currently prove value >= threshold. Some gates need a band (e.g. income between X and Y for a subsidy tier) without revealing the exact figure.

Scope

• Add a range mode (min <= value <= max) to income (and reuse the range_proof circuit if suitable), bound to the signed commitment.
• Wire thresholds through witness/SDK/UI; document the public input layout.
• Keep the existing threshold mode.

Value

Supports tiered eligibility, not just minimums.

Acceptance

• Income can be proven within a band with fixtures/VK/tests; threshold mode still works.

[Zuno2026]

@Zuno2026 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

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[Goodness-0x]

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I have reviewed this issue and understand the requirements.

I'd be happy to work on it. Please assign it to me.

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