feat: add typed CssBorderCollapse property support

Add CssBorderCollapse sealed class with separate/collapse keywords
and standard escape hatches (variable, raw, global). Wire it into
Style.typed() as borderCollapse parameter.

Closes #67

Author: kevin-sakemaer

packages/spark_css/CHANGELOG.md

@@ -10,6 +10,7 @@
 - **Feat**: Added `accent-color` CSS property support via `CssColor? accentColor` parameter in `Style.typed()`.
 - **Feat**: Added `CssAnimation` sealed class for the `animation` shorthand property with `.none`, single animation (name, duration, timingFunction, delay, iterationCount, direction, fillMode, playState), `.multiple()`, plus `.variable()`, `.raw()`, and `.global()` escape hatches.
 - **Feat**: Added `CssAnimationDirection`, `CssAnimationFillMode`, `CssAnimationPlayState`, and `CssAnimationIterationCount` sealed classes for animation sub-properties. Reuses existing `CssDuration` and `CssTimingFunction` from `CssTransition`.
+- **Feat**: Added `CssBorderCollapse` sealed class for `border-collapse` property with `.separate`, `.collapse` keywords, plus `.variable()`, `.raw()`, and `.global()` escape hatches.
 
 ### Changed
 

packages/spark_css/lib/src/css_types/css_border_collapse.dart

@@ -0,0 +1,55 @@
+import 'css_value.dart';
+
+/// CSS border-collapse property values.
+sealed class CssBorderCollapse implements CssValue {
+  const CssBorderCollapse._();
+
+  static const CssBorderCollapse separate = _CssBorderCollapseKeyword(
+    'separate',
+  );
+  static const CssBorderCollapse collapse = _CssBorderCollapseKeyword(
+    'collapse',
+  );
+
+  /// CSS variable reference.
+  factory CssBorderCollapse.variable(String varName) =
+      _CssBorderCollapseVariable;
+
+  /// Raw CSS value escape hatch.
+  factory CssBorderCollapse.raw(String value) = _CssBorderCollapseRaw;
+
+  /// Global keyword (inherit, initial, unset, revert).
+  factory CssBorderCollapse.global(CssGlobal global) = _CssBorderCollapseGlobal;
+}
+
+final class _CssBorderCollapseKeyword extends CssBorderCollapse {
+  final String keyword;
+  const _CssBorderCollapseKeyword(this.keyword) : super._();
+
+  @override
+  String toCss() => keyword;
+}
+
+final class _CssBorderCollapseVariable extends CssBorderCollapse {
+  final String varName;
+  const _CssBorderCollapseVariable(this.varName) : super._();
+
+  @override
+  String toCss() => 'var(--$varName)';
+}
+
+final class _CssBorderCollapseRaw extends CssBorderCollapse {
+  final String value;
+  const _CssBorderCollapseRaw(this.value) : super._();
+
+  @override
+  String toCss() => value;
+}
+
+final class _CssBorderCollapseGlobal extends CssBorderCollapse {
+  final CssGlobal global;
+  const _CssBorderCollapseGlobal(this.global) : super._();
+
+  @override
+  String toCss() => global.toCss();
+}

packages/spark_css/lib/src/css_types/css_types.dart

@@ -12,6 +12,7 @@ export 'css_background_position.dart';
 export 'css_background_repeat.dart';
 export 'css_background_size.dart';
 export 'css_border.dart';
+export 'css_border_collapse.dart';
 export 'css_border_radius.dart';
 export 'css_box_shadow.dart';
 export 'css_color.dart';

packages/spark_css/lib/src/style.dart

@@ -291,6 +291,7 @@ class Style implements CssStyle {
     CssBorder? borderBottom,
     CssBorder? borderLeft,
     CssBorderRadius? borderRadius,
+    CssBorderCollapse? borderCollapse,
     CssOutline? outline,
     CssLength? outlineOffset,
     // Visual
@@ -415,6 +416,9 @@ class Style implements CssStyle {
     if (borderRadius != null) {
       _properties['border-radius'] = borderRadius.toCss();
     }
+    if (borderCollapse != null) {
+      _properties['border-collapse'] = borderCollapse.toCss();
+    }
     if (outline != null) _properties['outline'] = outline.toCss();
     if (outlineOffset != null) {
       _properties['outline-offset'] = outlineOffset.toCss();

packages/spark_css/test/css_border_collapse_test.dart

@@ -0,0 +1,27 @@
+import 'package:spark_css/spark_css.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('CssBorderCollapse', () {
+    test('keywords output correct CSS', () {
+      expect(CssBorderCollapse.separate.toCss(), equals('separate'));
+      expect(CssBorderCollapse.collapse.toCss(), equals('collapse'));
+    });
+    test('variable outputs correct CSS', () {
+      expect(CssBorderCollapse.variable('bc').toCss(), equals('var(--bc)'));
+    });
+    test('raw outputs value as-is', () {
+      expect(CssBorderCollapse.raw('collapse').toCss(), equals('collapse'));
+    });
+    test('global outputs correct CSS', () {
+      expect(
+        CssBorderCollapse.global(CssGlobal.inherit).toCss(),
+        equals('inherit'),
+      );
+    });
+    test('Style.typed integration', () {
+      final style = Style.typed(borderCollapse: CssBorderCollapse.collapse);
+      expect(style.toCss(), contains('border-collapse: collapse;'));
+    });
+  });
+}