Add Swift Package helper methods and fix test runner issue

[EvanBacon]

Summary

• Adds convenient helper methods for working with Swift packages (addresses SwiftPackage elements not working correctly #31 point 2)
• Fixes pre-existing Bun test runner "export not found" error that prevented API tests from running

New Helper Methods

PBXProject:

• addPackageReference(ref) - Add a package reference to packageReferences
• getPackageReference(identifier) - Find package by URL (remote) or path (local)
• addRemoteSwiftPackage(opts) - Create and add remote package in one call
• addLocalSwiftPackage(opts) - Create and add local package in one call

PBXNativeTarget:

• addSwiftPackageProduct(opts) - Full wiring: creates product dependency, adds to target, creates build file, adds to frameworks phase
• getSwiftPackageProductDependencies() - Returns all package product deps for target
• removeSwiftPackageProduct(dep) - Removes product dep and cleans up build file

PBXBuildFile:

• createFromProductRef(opts) - Creates build file for Swift package products (no fileRef required)

Usage Example

const project = XcodeProject.open(xcodeProjectPath);
const target = project.rootObject.getMainAppTarget("ios");

// Add package to project
const packageRef = project.rootObject.addLocalSwiftPackage({
  relativePath: "../node_modules/react-native",
});

// Add product to target (handles all wiring automatically)
const productDep = target.addSwiftPackageProduct({
  productName: "React",
  package: packageRef,
});

Bug Fixes

• Made fileRef optional in PBXBuildFile JSON type (Swift package build files only have productRef)
• Fixed Bun test runner error by using export type { AnyBuildPhase } instead of value export
• Fixed circular dependency in utils/paths.ts by importing directly from source files

Test plan

• All 634 existing tests pass
• Added 50 new tests for Swift package functionality
• Manual verification of end-to-end workflow

Closes #31

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