Fix timer callback to dynamically lookup function

[briangu]

Summary

• make timer callback look up the function symbol each tick
• mention dynamic lookup in timer docs
• add a regression test for redefining timer functions

Testing

• python3 -m unittest

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https://chatgpt.com/codex/tasks/task_e_6871a2a752d48332858e4c2e87522805

[briangu]

#51