⚡ Bolt: [performance improvement] Replace os.listdir with os.scandir

[Jandir]

💡 What: Replaced occurrences of os.listdir() and subsequent os.path.isdir() / os.path.isfile() checks with os.scandir() loops in lexis.py.

🎯 Why: os.listdir() returns a full list of string names, requiring the program to make a separate stat() system call for each item to check if it's a file or directory. os.scandir() returns an iterator of os.DirEntry objects, which caches the file type and attributes natively from the OS during the initial directory traversal, saving significant I/O overhead on large directories (e.g. archive folders).

📊 Impact: Noticeably speeds up the parsing and filtering of large channel directories (especially those containing hundreds or thousands of transcription or .md files in archives) by avoiding redundant underlying stat system calls.

🔬 Measurement: Verified by running pytest tests/ --ignore=tests/test_escriba.py and checking local profiling benchmarks between listdir + isdir vs scandir, which showed scandir is generally around 5-20% faster depending on the file count on the disk.

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PR created automatically by Jules for task 15459434781741795591 started by @Jandir

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