Validate SCRAM server-final before authenticated SASL completion

Changes: Add an isolated Stanza 12.22.1 SASL feature replacement using existing SCRAM crypto, exact dependency lock and MIT notices, 44 integrated fixture checks, proposed ADR and review handoff. Leave production adapters and the original library experiment unchanged.
Validation: Fresh-cache npm ci and npm test passed on Node 24.21.0/npm 11.19.0; 44 tests cover RFC SHA-1/SHA-256 proofs and verified finals, stock defect reproduction, invalid/premature finals and async cleanup. make check passed 72 tooling tests. Pins/licenses/notices, unchanged vendor source and whitespace verified. Browser/WSS/server interoperability and constant-time assurance remain unverified; independent security review is pending.
