codify UI spacing constants (T-86)

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>

lib/widgets/widgets.dart

@@ -28,6 +28,7 @@ export 'src/multitab_pane.dart';
 export 'src/clide_tappable.dart';
 export 'src/clide_text.dart';
 export 'src/clide_tooltip.dart';
+export 'src/spacing.dart';
 export 'src/typography.dart';
 export 'src/icons/check.dart';
 export 'src/icons/chevron.dart';