Steadily dropping mortgage rates, which hit a record low 4.42 percent in numbers released this week, appear to be stimulating housing-market activity. Loan applications rose 13 percent in the week that ended last Friday, compared with the week before, says the Mortgage Bankers Association. But the low rates so far are generating more interest in refinancing existing loans than in home-purchase transactions. The association’s index of refinance activity rose 17 percent for the week, while an index of purchasing activity actually fell a bit.