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/multitab_pane.dart

@@ -4,6 +4,7 @@ import 'package:clide/widgets/src/clide_tappable.dart';
 import 'package:clide/widgets/src/clide_text.dart';
 import 'package:clide/widgets/src/icons/x.dart';
 import 'package:clide/widgets/src/multitab_controller.dart';
+import 'package:clide/widgets/src/spacing.dart';
 import 'package:flutter/widgets.dart';
 
 typedef MultitabBuilder<T> = Widget Function(BuildContext context, MultitabEntry<T> entry);
@@ -27,7 +28,7 @@ class MultitabPane<T> extends StatelessWidget {
     this.onAddRequested,
     this.allowReorder = true,
     this.keepAlive = false,
-    this.tabHeight = 28,
+    this.tabHeight = clideControlHeight,
   });
 
   final MultitabController<T> controller;
@@ -321,13 +322,13 @@ class _TabState<T> extends State<_Tab<T>> {
           builder: (context, _, __) => Container(
             constraints: BoxConstraints(minWidth: 96, maxWidth: 200),
             height: widget.tabHeight,
-            // Left margin stays at 12 (text breathing room).
-            // Right margin matches the close button's vertical
-            // breathing room ((tabHeight − iconSize) / 2 ≈ 6) so the
-            // gap around the icon is uniform on top, bottom, and right.
+            // Left: text inset (title breathing room).
+            // Right: when there's a close button, match its uniform
+            // icon margin so top/bottom/right are equal — see the
+            // ui-design `geometry.md` "no double-edge padding" rule.
             padding: EdgeInsets.only(
-              left: 12,
-              right: widget.onClose != null ? 6 : 12,
+              left: clideInsetText,
+              right: widget.onClose != null ? clideInsetIcon : clideInsetText,
             ),
             decoration: BoxDecoration(
               color: bg,
@@ -351,14 +352,14 @@ class _TabState<T> extends State<_Tab<T>> {
                 ),
                 // Right column: close icon, fixed natural width.
                 if (widget.onClose != null) ...[
-                  const SizedBox(width: 8),
+                  const SizedBox(width: clideGapStandard),
                   Opacity(
                     opacity: _hovered || widget.active ? 1.0 : 0.0,
                     child: ClideTappable(
                       onTap: widget.onClose,
                       builder: (context, hovered, _) => Container(
-                        width: 16,
-                        height: 16,
+                        width: clideIconHitTarget,
+                        height: clideIconHitTarget,
                         alignment: Alignment.center,
                         decoration: BoxDecoration(
                           color: hovered ? tokens.listItemHoverBackground : null,
@@ -366,7 +367,7 @@ class _TabState<T> extends State<_Tab<T>> {
                         ),
                         child: ClideIcon(
                           const CloseIcon(),
-                          size: 10,
+                          size: clideIconMicro,
                           color: hovered ? tokens.globalForeground : tokens.globalTextMuted,
                         ),
                       ),
@@ -397,14 +398,14 @@ class _AddButton extends StatelessWidget {
       child: ClideTappable(
         onTap: onTap,
         builder: (context, hovered, _) => Container(
-          width: 28,
+          width: clideControlHeight,
           height: tabHeight,
           alignment: Alignment.center,
           decoration: BoxDecoration(
             color: hovered ? tokens.listItemHoverBackground : null,
           ),
           child: ClideText('+',
-              fontSize: 14,
+              fontSize: clideIconStandard,
               color: hovered ? tokens.globalForeground : tokens.globalTextMuted),
         ),
       ),

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;