Adds lib/widgets/src/spacing.dart with three categories of named
constants — insets (clideInsetHairline / Tight / Icon / Standard /
Text), gaps (clideGapTight / Standard / Section / SectionLarge /
Major / Column), and sizes (clideIconMicro / Caption / Standard /
HitTarget / Emphatic, clideControlHeight).
Migrates MultitabPane to consume the constants and updates the
ui-design geometry reference to point at them. Inline pixel
literals in widget code were drifting (12 here, 6 there, 28
elsewhere) — pulling them through named symbols makes the
"uniform inner spacing" rule enforceable instead of eyeballed.
Co-Authored-By: Claude <noreply@anthropic.com>
jpmschweitzerandClaude
e329f1cc6f