rtf: single-pass prelude scan; cap group nesting depth

[jackoconner45]

RTF previously lexed the input five times just for the prelude and codepage; one scan now extracts both (~23% faster on a 4.5 MB RTF).

Group nesting depth was unbounded; it now caps at MAX_RTF_DEPTH (256) with a ResourceLimit error.

All tests pass; snapshot output unchanged.

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