Refactor LTML indexes and leader rendering

[rowland]

Summary

• Rework <index> to expand a repeated row template through normal container layout, with new index-title, index-page, and index-leader placeholders.
• Add a shared text-leaf core for paragraphs, labels, and sectors to reduce duplicated rich-text plumbing.
• Introduce a general inline <leader> primitive and refine default leader spacing so TOC numbers stay edge-aligned without feeling cramped.
• Normalize the public index tag spelling to <index-entry>, update docs and samples, and add new RTL and multi-page index coverage.

Testing

• Added and updated LTML unit tests for index parsing, leader wrapping, spacing, font overrides, and RTL behavior.
• Rendered updated samples locally to verify the new index layouts, including the Arabic index sample and the expanded TOC sample.
• go test ./ltml passes.