refactor: remove Foundation dependency from W3C CSSOM

Remove Foundation import from all CSSOM source files and replace
Foundation-specific functionality with native Swift equivalents:

- Replace String(format: "\\%x ", value) with String(_:radix:16)
  in escapeAsCodePoint() functions
- Remove unused Foundation imports from identifier and serialization files

All 134 tests pass with these changes. The package now uses only
native Swift functionality for string formatting and manipulation.

Modified files:
- Serialization/StringSerialization.swift
- Serialization/IdentifierSerialization.swift
- Serialization/Url.swift
- Serialization/String.swift
- Identifiers/DashedIdent.swift
- Identifiers/Ident.swift
- Identifiers/CustomIdent.swift

Author: coenttb

Sources/W3C CSSOM/Identifiers/CustomIdent.swift

@@ -1,5 +1,3 @@
-import Foundation
-
 /// Represents a CSS custom identifier for user-defined values.
 ///
 /// The `CustomIdent` data type denotes arbitrary user-defined strings used as identifiers

Sources/W3C CSSOM/Identifiers/DashedIdent.swift

@@ -1,5 +1,3 @@
-import Foundation
-
 /// Represents a CSS dashed identifier for user-defined custom properties and values.
 ///
 /// The `DashedIdent` data type denotes arbitrary user-defined strings that start with two dashes

Sources/W3C CSSOM/Identifiers/Ident.swift

@@ -1,5 +1,3 @@
-import Foundation
-
 /// Represents a CSS identifier string.
 ///
 /// The `Ident` type represents an arbitrary string used as an identifier in CSS.

Sources/W3C CSSOM/Serialization/IdentifierSerialization.swift

@@ -1,5 +1,3 @@
-import Foundation
-
 /// Serializes an identifier according to the CSSOM specification.
 ///
 /// This function implements the identifier serialization algorithm defined in
@@ -118,5 +116,5 @@ private func isValidIdentifierCharacter(_ scalar: Unicode.Scalar) -> Bool {
 /// - U+001F → `\1f `
 /// - U+007F → `\7f `
 private func escapeAsCodePoint(_ scalar: Unicode.Scalar) -> String {
-    return String(format: "\\%x ", scalar.value)
+    return "\\\(String(scalar.value, radix: 16)) "
 }

Sources/W3C CSSOM/Serialization/String.swift

@@ -1,5 +1,3 @@
-import Foundation
-
 /// Represents a CSS string value.
 ///
 /// The `CSSString` type represents a sequence of characters used in CSS properties

Sources/W3C CSSOM/Serialization/StringSerialization.swift

@@ -1,5 +1,3 @@
-import Foundation
-
 /// Serializes a string according to the CSSOM specification.
 ///
 /// This function implements the string serialization algorithm defined in
@@ -81,5 +79,5 @@ func serializeString(_ string: String) -> String {
 /// - U+001F → `\1f `
 /// - U+007F → `\7f `
 private func escapeAsCodePoint(_ scalar: Unicode.Scalar) -> String {
-    return String(format: "\\%x ", scalar.value)
+    return "\\\(String(scalar.value, radix: 16)) "
 }

Sources/W3C CSSOM/Serialization/Url.swift

@@ -1,5 +1,3 @@
-import Foundation
-
 /// Represents a CSS URL value.
 ///
 /// The `Url` type is a pointer to a resource such as an image, video, CSS file, font file, or SVG.