Design: interaction ergonomics, measured from code

What this subtree is for. The design syllabus queue: A-line interaction ergonomics (Fitts, Hick, response-time thresholds, steering law, working memory and chunking, thumb zone, direct manipulation, pen ergonomics, error ergonomics, Jakob, serial position, Tesler), B-line visual and layout, C-line evaluation methods. Each entry is a closed, testable item: a mechanism plus its cause, not a tip.

Working rules for this subtree.

• Measure from repo code, not rendered impression: sizes, counts, and breakpoints come from the design-system repo and the consuming shell's actual usage, per device and per breakpoint (desktop wide, tablet, phone, desktop shell, browser plugin are separate sets of numbers).
• Depth parity with the SWE fundamentals shelf: a formula comes with its derivation and a number comes with its origin experiment. Shallow "design tips" are not accepted.
• Every ergonomics rule is stated with its scope: which devices it holds for, where it breaks, and which other rule it trades against.