Shake lime juice, mezcal, amaro, agave, and egg white in a cocktail shaker (or in a tightly lidded 1-qt. glass jar) until frothy, about 1 minute. Fill shaker with ice, cover, and shake again until outside is frosty, about 30 seconds. Strain through a fine-mesh sieve into a coupe glass. Garnish with a few drops of Angostura bitters (drag a toothpick through drops to create a swirly pattern if you wish). Top with lime twist.