Protect log handlers from dangling message buffers

[imakris]

Summary

• add a thread-local log message ring that copies text before invoking handlers so we never hand out pointers to destroyed buffers and return the stabilized pointer from glatter_log
• extend the logging unit test to assert that heap-allocated messages are copied and preserved for the handler

Testing

• cmake -S . -B build (fails: missing OpenGL libraries on build image)

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https://chatgpt.com/codex/tasks/task_b_68f2952a4e24832dba89bf092dab5ebb