resolveInlineStep

Description#

Resolve a horizontal arrow key into an inline-axis index step, honouring the writing direction. Under ltr, ArrowRight advances (+1) and ArrowLeft retreats (-1); under rtl the two swap, because the first item in reading order sits on the right. Any non-horizontal key (including ArrowUp / ArrowDown / Home / End) returns null - the block axis is the caller's concern and never direction-aware.

This is the mechanical kernel every direction-aware keyboard strategy shares (roving, tabs, stepper strip, slider, reorder); it is a pure function, not a DI chokepoint - each strategy injects its own direction and calls this.