Simplify ad mode detection logic

[paulpv]

Summary

• replace the timing-based scheduling with simple counter thresholds for entering/exiting ad mode
• add ad/program segment counters and reset logic tied to the new thresholds
• drop the old segment duration measurement helpers that are no longer used

Testing

• not run (extension has no automated tests)

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https://chatgpt.com/codex/tasks/task_e_68c90a8aa94c8333beefd69f1f46843e