Complete missing implementations and add comprehensive test infrastructure

[Copilot]

Codebase had compilation errors, incomplete stubs, and minimal test coverage. This PR completes the missing implementations and establishes a solid testing foundation.

Core API Completeness

• Added bytes_transferred field to Request struct for I/O completion tracking
• Implemented Queue::take_completed() method with proper mutex synchronization
• Returns snapshot of completed requests, clears internal buffer atomically

std::vector<Request> take_completed() {
    std::lock_guard<std::mutex> lock(mtx_);
    std::vector<Request> result;
    result.swap(completed_);
    return result;
}

Compression & Error Handling

• GDeflate compression now reports ENOTSUP via error callback instead of silent failure
• Restructured compression handling to explicitly check compression modes
• Added proper error context population for unsupported operations

Enum & C ABI Updates

• Added RequestStatus::Cancelled enum value for future cancellation support
• Synchronized C ABI: added DS_REQUEST_CANCELLED and DS_COMPRESSION_GDEFLATE
• Updated all enum conversion functions between C and C++ APIs
• Added bytes_transferred to C request struct

Test Infrastructure

Added 4 test suites covering previously untested code paths:

• cpu_backend_test: Read/write, partial reads, compression, concurrent operations
• error_handling_test: Invalid FD, missing files, error context validation
• gdeflate_stub_test: Unsupported compression error verification
• basic_queue_test: Core queue operations (existing, now integrated)

CI/CD & Documentation

• GitHub Actions workflow: GCC + Clang × Debug + Release matrix builds
• Updated README to reflect actual working state, removed stale "build broken" warnings
• Documented known limitations (GDeflate spec needed, Vulkan compute pending)

Build Status: All compilation errors resolved, 4/4 test suites passing

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