stepper: the step slider counts from 0

[martin-henz]

The stepper's step slider now counts the steps taken from 0, like the e-stepper's and the CSE machine's: 0 is the start, the right end is the number of steps taken, and both ends are labelled (Blueprint's own labelling stopped short of the end). The label helper is shared with the e-stepper (sliderLabels, now in its own module so the e-stepper does not pull in the stepper's hotkey library).

• A run of one step gets a valid disabled range (0 to 1).
• The welcome text's even/odd note follows the numbering: odd-numbered steps highlight the next redex in yellow, even-numbered steps (after 0) the result in green.

The step limit now means the same in all three tools (the number at the right end never exceeds it): source-academy/py-slang#529.

Tests: the range, value and labels for a long run; stepping and the double arrows; a run of one step; the label values.

🤖 Generated with Claude Code

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