Add lazy direct child index

[iDavi]

Summary

• build a shared direct-child token-id buffer only after repeated indexed lookup on a container with 32+ children
• return indexed children directly after the lazy index is built
• reset lazy-index storage when loading a new document

Why

FindIndexChild traversed sibling links from the first child on every call. A complete indexed scan therefore became quadratic.

Validation

• reviewed index offsets, growth, and reset lifecycle statically
• no VBA runtime or test suite is available in this environment

Closes #3