Missing copy constraint in halo2_gadgets variable-base scalar multiplication allows under-constrained base, breaking Orchard Action circuit soundness

ZEBRA is a Zcash node written entirely in Rust. Prior to zebrad 5.0.0, halo2_gadgets 0.5.0, orchard 0.14.0, zcash_primitives 0.28.0, and zcashd 6.20.0, the variable-base scalar multiplication gadget in halo2_gadgets/src/ecc/chip/mul/incomplete.rs used assign_advice() for the base point without a copy constraint tying it to the actual base, allowing a malicious prover to produce a valid proof for an Orchard Action with an under-constrained base point and bypass the diversified-address-integrity check that binds pk_d, g_d, ivk, the nullifier (nf), and the spend validating key (ak) to the note being spent. This issue is fixed in zebrad 5.0.0, halo2_gadgets 0.5.0, orchard 0.14.0, zcash_primitives 0.28.0, and zcashd 6.20.0.