Feature: date_range circuit — prove a credential was issued within a validity window #390

Some gates care not just about a threshold but about recency: prove that a credential (e.g. a KYC re-check) was issued within the last N days without revealing the exact issuance date.

Scope

• New Noir circuit that takes a private issuance_date and public (current_date, max_age_days), asserting current_date - issuance_date <= max_age_days, bound to the signed commitment.
• Keep the shared commitment + ECDSA discipline (non-zero salt, Poseidon2) consistent with the other circuits.
• Register a VK, add fixtures, and document the public input layout.

Value

Enables freshness-based gating (recent KYC, recent proof-of-funds) that the current threshold-only circuits cannot express.

Acceptance

• The circuit proves recency within a window, has fixtures + VK + tests, and is wired through witness/proof/SDK like existing types.