Passing both --fixed and --target-pages silently ignores --target-pages

[adisakshya]

Context

split_book.py, main().

Problem

If a user passes both --fixed N and --target-pages M, --fixed silently wins (if args.fixed is not None: ...) and --target-pages is dropped with no indication to the user that it was ignored.

Reproduction

1. python split_book.py book.pdf --fixed 3 --target-pages 100 --list
2. Output is a plain fixed-window plan; nothing mentions that --target-pages 100 was ignored. Confirmed.

Expected

Either reject the combination with a clear error ("--fixed and --target-pages are mutually exclusive"), or print an explicit note that --target-pages is being ignored because --fixed takes precedence.

Actual

Silent, undocumented precedence with no feedback.

Acceptance criteria

• Passing both flags together produces either a clear rejection or an explicit stdout/stderr note about which one is being used.
• Regression test added.
• README/--help text documents the interaction.