> To whit, the few retail equity US and Canadian trading platforms I've used (Schwab's, Interactive Brokers, Scottrade before they became TD Ameritrade, Questrade) also do not provide an explicit feature that purports to calculate capital gains

I cannot account for the discrepancy, but I use Charles Schwab, and it absolutely DOES calculate and display capital gains. In cases where I have moved a security into the account (rather than purchasing it directly within the account) I have had to provide original cost basis information for them in order for the calculations to be correct.