The optimal block splitter stores up to 196 split points and then needs one
additional partition entry for the terminal sequence boundary consumed by the
C block-emission loop. The previous array declaration provided only the split
slots, so a maximally partitioned block could write the terminal boundary past
the projected Rust/C state. Reserve the extra entry and document the layout;
retain focused tests for equal-cost and estimation-error cases so the splitter
continues to avoid unnecessary or invalid partitions.
Test Plan:
- Focused block-split Rust tests -- 7 passed
- `make -B -C lib -j2 lib` -- passed
- `cargo +nightly fmt --manifest-path rust/Cargo.toml -- --check` -- passed
- Scoped `git diff --check` -- passed
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