opus_flutter_ios: add arm64-simulator slice to opus.xcframework

[gfx]

Summary

• opus.xcframework previously only shipped ios-arm64 (device) and ios-x86_64-simulator (Intel simulator) slices — no arm64-simulator slice.
• On Apple Silicon Macs, Xcode defaults to building the simulator target as arm64, so any app linking this framework fails to build for the simulator with Unable to find matching slice in 'ios-arm64 ios-x86_64-simulator' for the current build architectures (arm64 x86_64) ....
• Cross-compiled libopus 1.3.1 (portable C, float API — matches the existing build: no FIXED_POINT, no custom modes, no arch-specific assembly) for arm64-apple-ios13.0-simulator from the vendored source already in this repo (opus_flutter_android/android/libopus).
• Merged the new arm64 binary with the existing x86_64 binary via lipo into a single fat ios-arm64_x86_64-simulator slice — the same layout xcodebuild -create-xcframework produces for multi-arch simulator support — and updated Info.plist accordingly.

Verification

• Symbol table of the new arm64 binary matches the existing x86_64 binary exactly (including the auto-generated opusVersionString/opusVersionNumber framework version symbols).
• Built and ran libopus's own test suite (test_opus_api, test_opus_decode, test_opus_encode) natively on arm64 with the exact same source/flags — all pass with zero failures (this is a valid proxy for the iOS-Simulator arm64 build since Apple Silicon iOS Simulator processes execute unmodified arm64 code on the host CPU; only SDK/platform metadata differs from a native macOS-arm64 build for this platform-API-free C code).
• Built the opus_flutter example app (with opus_flutter_ios pointed at this change) against this xcframework and ran it, unmodified, on an iOS 26 (arm64-only) Simulator runtime on an Apple Silicon Mac — confirmed both the Runner binary and the embedded opus.framework binary are natively arm64 (no x86_64, no Rosetta) via file/otool.

Test plan

• Symbol parity check between new arm64 slice and existing x86_64 slice
• libopus API/encode/decode test suite passes on arm64
• opus_flutter example app builds and launches on an arm64-only iOS Simulator runtime, natively
• Maintainer review of the build flags / xcframework structure

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