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/src/spacing.dart

@@ -0,0 +1,89 @@
+/// Spacing constants shared across widgets.
+///
+/// Three categories — every widget that picks a literal pixel value
+/// for layout should pull it from one of these instead:
+///
+/// 1. **Insets** — padding inside containers around content
+///    (text breathing room, icon micro-margin).
+/// 2. **Gaps** — distance between siblings in a Row or Column.
+/// 3. **Sizes** — control dimensions (icon glyph size, hit-target
+///    outer size, button/row/tab height).
+///
+/// Values are paired with the geometry rules under
+/// `.claude/skills/ui-design/references/geometry.md` (uniform inner
+/// spacing, no double-edge padding, two-column control pattern).
+/// Adjust here only — never inline literals.
+library;
+
+// ---------------------------------------------------------------------------
+// Insets — padding inside containers
+// ---------------------------------------------------------------------------
+
+/// Hairline. Used for divider thickness and the terminal cell padding
+/// where 1px would clip glyphs.
+const double clideInsetHairline = 2;
+
+/// Tight inset for compact controls (sidebar item gutter).
+const double clideInsetTight = 4;
+
+/// Uniform breathing room around an icon inside a control. Pair with
+/// a 16x16 hit target inside a 28-tall control: top/bottom auto-fall
+/// to (28 − 16) / 2 = 6, so a matching right padding of 6 yields a
+/// uniform border on the constrained sides.
+const double clideInsetIcon = 6;
+
+/// Standard gap between adjacent inline elements in a row (icon → text,
+/// text → action). Used as the in-row SizedBox between a title and its
+/// trailing close button.
+const double clideInsetStandard = 8;
+
+/// Text-content padding. Used as the leading horizontal padding of
+/// text-bearing controls (tab title left padding, list-item gutter).
+const double clideInsetText = 12;
+
+// ---------------------------------------------------------------------------
+// Gaps — distance between siblings
+// ---------------------------------------------------------------------------
+
+/// Tight gap between row segments inside a control.
+const double clideGapTight = 4;
+
+/// Standard gap between sibling controls (between a title and an
+/// action icon, between two list items in a flex row).
+const double clideGapStandard = 8;
+
+/// Section internal gap (between a section header and its first row).
+const double clideGapSection = 14;
+
+/// Section gap (between two sections in a stack).
+const double clideGapSectionLarge = 20;
+
+/// Major gap between large blocks (between two columns in a row,
+/// between header and content card).
+const double clideGapMajor = 24;
+
+/// Welcome-screen-style gap between the two centered columns.
+const double clideGapColumn = 56;
+
+// ---------------------------------------------------------------------------
+// Sizes — control dimensions
+// ---------------------------------------------------------------------------
+
+/// Micro icon glyph (close ×, dropdown chevron when paired with text).
+const double clideIconMicro = 10;
+
+/// Caption-row icon (status bar, sidebar inline icons).
+const double clideIconCaption = 13;
+
+/// Standard inline icon (icon rail, action buttons).
+const double clideIconStandard = 14;
+
+/// Hit-target outer container around a micro icon. Provides hover
+/// background and a comfortable click area; the icon centers inside.
+const double clideIconHitTarget = 16;
+
+/// Emphatic icon (standalone affordances, primary action glyphs).
+const double clideIconEmphatic = 18;
+
+/// Standard control height (tab, button, list row).
+const double clideControlHeight = 28;