metal: convert shutdown assertion to warning log

[arkavo-com]

Summary

During process exit, Metal residency sets may not be fully cleaned up due to static destructor ordering. This converts the fatal assertion in ggml_metal_rsets_free() to a warning log, as the OS will reclaim GPU resources anyway.

Problem

When embedding llama.cpp in applications that use exit(), receive SIGINT, or don't perfectly clean up all contexts before shutdown, the assertion at line 608 fires:

GGML_ASSERT([rsets->data count] == 0) failed

This happens because:

1. The Metal device is a singleton (static unique_ptr in ggml_metal_device_get())
2. It's only freed via C++ static destructor during process exit
3. Application-level cleanup may not run (e.g., Rust's Drop impls are skipped when std::process::exit() is called)

Solution

Replace the assertion with a warning log. This:

• Preserves the diagnostic value (developers can see if resources weren't freed)
• Doesn't crash applications during normal shutdown
• Is safe because the OS reclaims all GPU resources on process exit

Test

1. Build llama.cpp with Metal enabled
2. Run any application that loads a model
3. Send SIGINT (Ctrl+C) during or after inference
4. Verify clean shutdown with warning instead of assertion crash

[arkavo-com]

Closing this PR as it was AI-generated, which violates the project's contributing guidelines. The fix will be resubmitted by a human contributor.