EPUB: split large XHTML flows and add MOBI page-break option

[M-Kepler]

Closes #31.

Summary

• Split oversized EPUB XHTML at safe element boundaries, including <br> boundaries inside a single large paragraph, while relocating links and TOC targets.
• Match Calibre's CLI default --flow-size of 260 KiB for EPUB output. --flow-size N changes the limit, and --flow-size 0 disables size-based splitting.
• Add --dont-split-on-page-breaks for MOBI input. It suppresses source page-break and NCX-position splits while leaving size-based splitting active. Without the flag, existing source page-break handling remains enabled.

The size limit is best effort where content cannot be split safely.

Validation

• cargo test --locked
• cargo clippy --locked --all-targets -- -D warnings
• cargo fmt --check and git diff --check
• Converted a 29.6 MB MOBI with one large paragraph and 114,991 line breaks. Both default conversion and --dont-split-on-page-breaks produced 53 XHTML files, largest 266,239 bytes (below 260 KiB). --flow-size 0 produced one XHTML file. All three outputs had identical body text hashes and line-break counts; ZIP, XML, and internal references checked clean.