--level 0 / negative --level silently produces a misleading "no usable outline" error instead of being rejected

[adisakshya]

Context

split_book.py, main() / build_arg_parser(). Compare to --fixed/--target-pages, which are already validated as positive integers with a clear error message.

Problem

--level has no validation. Internally, _walk_outline starts depth counting at 1, so --level 0 or any negative value can never match any outline entry — it silently falls through to the generic "no usable outline found at --level N" error, even on a PDF that has a perfectly good outline.

Reproduction

1. Use any PDF with a normal, working outline at level 1.
2. python split_book.py book.pdf --level 0 --list
3. Output: Error: no usable outline found at --level 0. Try a different --level (e.g. --level 1), ... — despite the outline existing and being perfectly usable at level 1. Confirmed.

Expected

--level should be validated as a positive integer up front (same pattern already used for --fixed/--target-pages in main()), with a message like: Error: --level must be a positive integer, got 0.

Actual

No validation; produces a confusing generic "no outline found" message instead.

Acceptance criteria

• --level 0 and negative --level values are rejected with a clear, specific error before any outline parsing is attempted.
• Regression test(s) added, mirroring the existing --fixed/--target-pages validation tests.