fix: Xcode 26 compatibility — strip .Swift submodule from RiveRuntime modulemaps

[mfazekas]

Fixes #173

RiveRuntime's prebuilt XCFramework was built with Swift 6.1. Xcode 26 uses Swift 6.2. Both generate .Swift Clang submodules with different C++ interop type definitions (swift::Optional, swift::String, etc), causing hard ODR errors that can't be suppressed. This is CocoaPods-specific — SPM is unaffected because it uses .swiftmodule files directly.

The fix hooks into CocoaPods' pre_install via Module#prepend in the podspec to strip the .Swift submodule from RiveRuntime's prebuilt modulemaps. This is transparent to users — no Podfile changes needed.

Modulemap patch

 framework module RiveRuntime {
   umbrella header "RiveRuntime.h"
   export *

   module * { export * }
 }
-
-module RiveRuntime.Swift {
-  header "RiveRuntime-Swift.h"
-  requires objc
-}

Build error without fix (Xcode 26.2)

RiveRuntime-Swift.h:3603:62: error: '(lambda ...)' has different definitions in different modules;
  first difference is definition in module 'RiveRuntime.Swift' found method 'operator()' with body
RiveRuntime-Swift.h:3596:78: error: 'swift::Optional::init' has different definitions in different modules;
  definition in module 'RiveRuntime.Swift' first difference is 1st parameter with name 'some_'
LocalCxxPod-Swift.h:2177:29: error: 'swift::UTF8View' has different definitions in different modules
LocalCxxPod-Swift.h:1984:37: error: 'swift::String' has different definitions in different modules
LocalCxxPod-Swift.h:1722:52: error: 'swift::Optional' has different definitions in different modules;
  first difference is definition in module 'LocalCxxPod.Swift' found method 'init' with 1st parameter named 'value'

After fix

BUILD SUCCEEDED (Xcode 26.2, clean DerivedData, pod install + xcodebuild)