FIRST_PAGE_INDEX

The index a blank document's first page gets.

a1, not ZERO_INDEX_KEY — every .tldr written by tldraw puts its first page at a1, including the reference fixture this project compares against, and a page seeded at a0 sorts before all of them. With a single page that is invisible; it shows up the moment two documents are merged or synced.

This is deliberately only about the SEEDED page. The index helpers still start at ZERO_INDEX_KEY and are internally consistent with each other; reconciling the generator itself with tldraw's first key is a change to how every index is allocated and does not belong here.