[parley] Limit input text size to 2^30 bytes

Nobody has claimed this yet.

1 GB of text per layout is more than generous. We're likely to see severe performance degradation long before reaching this limit anyway suggesting that applications wanting to support excessively large buffers will need to do some segmentation and caching.

Rationale: this lets us use `u32` instead of `usize` for internal indices which can significantly reduce memory requirements for internal data structures. Using 30 bits instead of 32 (or 31) allows us to occasionally steal a bit for other info (often incredibly useful) while still preserving the use of `MAX_SIZE + 1` as the exclusive end boundary.

The issue names no files or tests. Start by tracing parley’s layout input and internal index types, then determine where the 2^30-byte limit belongs. Done means the limit is enforced and the internal representation can use u32 while preserving the stated exclusive-boundary behavior.

Written by the indexing model from the issue text.