chore: add Swift code coverage via LLVM profiling

[mfazekas]

Summary

• Opt-in Swift code coverage for ios/ files using LLVM source-based profiling
• Activated by setting RIVE_SWIFT_COVERAGE=1 before pod install
• Produces lcov and HTML reports filtered to only library Swift code (excludes Pods, nitrogen, etc.)

Usage

cd example/ios && RIVE_SWIFT_COVERAGE=1 bundle exec pod install && cd ../..
# build & run harness tests, then:
bash scripts/ios-coverage.sh

How it works

• RNRive.podspec conditionally adds -profile-generate -profile-coverage-mapping to the RNRive pod
• CoverageHelper.swift flushes LLVM profraw data via __llvm_profile_write_file on a 1s background timer, SIGTERM, and app lifecycle events
• CoverageSetup.m bootstraps CoverageHelper via ObjC +load (no-op when coverage is disabled)
• scripts/ios-coverage.sh merges per-process profraw files and generates reports via llvm-cov
• Handles Xcode 26 debug.dylib (where code + coverage data lives in the dylib, not the main binary)

Test plan

• Verified locally: 7/7 harness test suites pass, 28.9% line coverage across 34 Swift files
• Non-coverage builds unaffected (pod install without env var compiles out all coverage code)