(Circuit) Pin q_init points to constants#246
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This PR adds `NonIdentityPoint::new_from_constant` to enable creating non-identity points that are properly pinned to constants, which is required by the Orchard ZSA circuit ([Orchard PR](QED-it/orchard#246)). More precisely, in the Orchard ZSA circuit, `q_init_zec` / `q_init_zsa` must be constrained to fixed constants, as they define the initial point `Q` of the Sinsemilla hash. If constructed via `NonIdentityPoint::new`, they remain unconstrained witnesses, allowing a prover to inject an arbitrary on-curve point and break commitment soundness.
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This PR fixes an issue in the ZSA note commitment circuit where
q_init_zecandq_init_zsawere previously treated as unconstrained witnesses. The fix pins these points to their expected constant values, ensuring they cannot be altered during proof generation.