test(spop): cover frame size handshake

[fionera]

Summary

• make the fake SPOP peer complete HELLO before sending NOTIFY
• use the mandatory HELLO fields in the fixture
• characterize offers at 65,535, 262,140, and math.MaxUint32
• verify AGENT-HELLO returns the accepted max-frame-size
• join peer and protocol goroutines so handshake failures cannot leak past the test

Why

This isolates the handshake coverage before changing frame-buffer sizing. Production behavior is unchanged here. The implementation follows separately in #33.

Validation

• go test ./...
• go test -race ./...
• repository custom vet analyzer
• staticcheck ./...