Experimental C++ version of InputSource and company

[sbooth]

Only std::exception subclasses are thrown.

[Copilot]

Pull Request Overview

This PR refactors the InputSource implementation from Objective-C to C++, introducing a new C++ class hierarchy with Objective-C wrappers. The main changes consolidate multiple Objective-C subclasses into a unified C++ implementation using std::expected for error handling.

Key changes:

• Replaces Objective-C subclasses (SFBDataInputSource, SFBFileInputSource, etc.) with C++ implementations
• Introduces new C++ base class InputSource with concrete implementations: DataInput, FileInput, FileContentsInput, and MemoryMappedFileInput
• Updates SFBInputSource to act as a wrapper around C++ implementations

Reviewed Changes

Copilot reviewed 23 out of 23 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
Package.swift	Updates C++ standard from C++20 to C++23 to support std::expected
scope_exit.hpp	Adds RAII helper for cleanup operations
InputSource.hpp/cpp	New C++ base class with virtual interface
DataInput.hpp/cpp	C++ implementation for data-based input sources
FileInput.hpp/cpp	C++ implementation for file-based input sources
FileContentsInput.hpp/cpp	C++ implementation for loading entire file contents
MemoryMappedFileInput.hpp/cpp	C++ implementation for memory-mapped files
SFBInputSource.mm	Refactored to wrap C++ implementations
SFBInputSource+Internal.h	Updated to store C++ unique_ptr instead of URL
SFB*InputSource.h/m (deleted)	Removed old Objective-C implementations
SFBAudioEngineTests.swift	Updated test to verify initial state

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