Add Stablecoin privacy-preserving tests for WithdrawCollateral and RepayDebt
(personal and group-owned variants), plus a regression test confirming
OpenPosition is incompatible with the privacy circuit (chained-call
re-authorization). Close the last planned Token row (MintWithAuthority to a
private holding) and the ATA owner-signer gap for Transfer (personal and
group-owned), plus a defensive Create/group-owner test.
Extract shared privacy-test helpers (identity builders, GroupOwner
seal/unseal handshake) into integration_tests/src/lib.rs and use them
throughout token.rs, collapsing duplicated InputAccountIdentity/account
construction. Update docs/privacy-test-matrix.md with all new findings.
Marvin Jones