[wip]refactor: introduce AppContext to replace global CancellationToken

[Itsusinn]

What does this PR do?

• Removes the global SHUTDOWN_TOKEN static variable and shutdown() function from clash-lib/src/lib.rs.
• Introduces an AppContext struct in clash-lib/src/app/context/mod.rs to properly manage lifecycle and cancellation signals across components.
• Propagates AppContext via dependency injection to core runtime components (InboundManager, TunRunner, DnsRunner, ApiRunner), replacing their internal standalone cancellation tokens.
• Replaces multiple instances of the custom print_and_exit! macro and direct std::process::exit calls (e.g. in config rule parsing, resolver initialization, and outbound manager) with returning Result::Err(crate::Error) for graceful error propagation, which is vital for using clash-lib as a robust library.

This provides a cleaner, more idiomatic architecture that prevents sudden process crashes on configuration errors and gives the library consumer full control over component lifecycle.

[github-actions]

🚀 TUN Throughput Benchmark Results

📊 Test Environment

Component	Details
OS	Linux 6.14.0-1017-azure
Architecture	x86_64
CPU	AMD EPYC 7763 64-Core Processor
CPU Cores	4
Memory	15.62 GB
Kernel	6.14.0-1017-azure
Network Interfaces	eth0 (50000 Mbps), docker0, enP52666s1 (50000 Mbps)

❌ Benchmark Failed

The TUN benchmark test failed to complete. Check the workflow logs for details.

Baseline test succeeded:

• Throughput: 34.79 Gbps

Common causes:

• TUN device initialization failure
• Network configuration issues
• Timeout during iperf3 test

_{🤖 Benchmark run on GitHub Actions CI •
Test duration: 10s • Linux x86_64 • 4 cores}