EQL bindings: generate the v4 Rust types, and derive Stack Encrypt's traits on v4 only #1179

Part of #1142, which keeps step 1 (publish @cipherstash/eql 4.0 with both bundles) and steps 3, 5 and 6.

Problem

Stack Encrypt's one producible type, TextEq, writes eql_v3_* domains. The eql_v3 TextEq and TextEqQuery structs derive Stack Encrypt's EncryptFrom (and TextEq its DecryptInto), but Stack Encrypt's output is not byte-for-byte compatible with cipherstash-client's. So those derives claim a v3 compatibility that doesn't exist. ADR-0002 (#1139) gives Stack Encrypt its own eql_v4 domains instead.

Proposal

#1142 steps 2 and 4:

• eql-codegen generates the scalar domains into eql_bindings::v4 (eql_v4_* names, a SchemaVersion pinned to 4, payloads with "v": 4).
• Only v4 structs derive EncryptFrom and DecryptInto. TextEq and TextEqQuery move to v4, and a test asserts that no v3 struct implements either trait.
• The Stack Encrypt target table moves from v3/targets.rs to v4/targets.rs, and the v3 targets and their "producer profile" reasons are deleted.
• The stack-encrypt:1: ciphertext prefix stays.
• Go's encrypt/eql/eql_gen.go is regenerated against v4.

The PostgreSQL test for the encryption crate runs against an eql_v4 install.