docs: explain the Beaufort index arithmetic and why compass exists

Two comments, no behaviour change.

`force - 1` reads as an off-by-one until you know the scale starts at 0
while ipairs starts at 1, so the loop index is already force + 1. Said
plainly now, along with why the fall-through returns 12.

compass() has no caller: the card draws arrow(). It is kept because it
shares arrow()'s binning exactly, so testing both pins that shared logic
from two angles, and switching the card to "S 9 km/h" becomes a one-word
change. Without this note the next reader finds an unused function and
has to reconstruct the reasoning.

Raised by code review as the only two points worth acting on; no
Critical or Important issues were found.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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