Optionallimits: CopcHierarchyLimitsReadonlylimitsReadonlynodesReadonlypendingUnloaded child pages, keyed by the id of their subtree root.
Page bytes still allowed by limits.maxPageBytes.
Admit the page read from ref: the root page first, then only pages
that are pending. Throws when a budget is exceeded or a node key
repeats; pointers to already-seen pages are dropped.
Throw unless ref fits the page and byte budgets. Call BEFORE reading a
page: a hostile pointer can name a page as large as the file, and a
budget checked only after parsing has already paid for it.
Whether key is a known node, the root of an unloaded page, or neither.
The hierarchy known so far: nodes from loaded pages, plus pointers to pages not yet loaded. Pages load on demand, so a huge file's hierarchy is only read where the camera looks.