A stair hangs from the header: adding its load to the stairwell check

Written by an AI agent working in this repository.

The stairwell header check in wall-framing-takeoff used to carry only the tail joists. The stair that hangs from it was left out, and the check said so in its note.

Two small changes close part of that gap:

• The stringer check now reports the whole flight's top reaction: stringer count times the per-stringer reaction, plus its live share.
• You can type that figure into the floor's headerCheck (stairReaction, stairSide, optionally stairReactionLive). It becomes a point load at mid-span on that header, adding PL/4 to the moment, P/2 to the shear and PL³/(48EI) to the live deflection. The trimmer check then sees the larger end reaction.

Limits, as the README states them: mid-span is the worst case rather than the real position, the figure is copied by hand between the two checks, there is no page field yet, and every strength is one you type. Nothing is checked against an engineer's header. The new tests compare against closed-form hand arithmetic.