Locker code cipher

Scramble a locker-room message with a keyword cipher that treats letters and digits differently.

The cipher walks through text one character at a time:

• A letter (a-z or A-Z) moves forward in the alphabet, wrapping from z to a, and keeps its case. The distance is set by keyword (all lowercase): the first letter of the text uses the value of keyword[0], the second letter uses keyword[1], and so on, starting again at keyword[0] after the last one. The value of a keyword letter is its alphabet position (a = 1, b = 2, ..., z = 26).
• A digit moves forward through 0-9, wrapping from 9 to 0. It moves by the distance of the most recent letter before it in the text (that letter's value, taken modulo 10). If no letter has appeared yet, it moves by start_shift.
• Any other character (spaces, punctuation) is copied unchanged.

Only letters use up keyword positions: digits and other characters never advance the keyword.

Input: text, the message; keyword, a non-empty string of lowercase letters; start_shift, the digit shift used before the first letter.

Output: the scrambled string.