Keep the LDM limit-table update on the Rust compression path while leaving
match-state ownership in C. Previously, the C block wrapper combined pointer
subtraction, match-state access, and the bounded scalar update. The wrapper
now computes `curr` from `anchor - window.base`, passes `curr` and
`nextToUpdate` through the narrow U32 ABI, and stores Rust's result before the
existing fast-table dispatch.
The Rust leaf uses explicit wrapping arithmetic to preserve the C U32 behavior:
the strict `curr > nextToUpdate + 1024` threshold and the `MIN(512, ...)`
clamp. Focused tests cover the threshold, one-step update, clamp, nonzero
starting point, and arithmetic wraparound.
Test Plan:
- `cargo test zstd_ldm` -- default-feature test-binary link failed because
existing dict-builder C symbols are not linked.
- `cargo test --no-default-features --features compression zstd_ldm` -- passed
(7 tests).
- `make lib-nomt` -- passed.
- `make lib-mt` -- passed.
- `make -C tests test-zstream` -- passed; it emitted the existing
`tests/zstreamtest.c` unterminated-string warning.
- `cargo clippy`, `cargo clippy --benches`, `cargo clippy --tests`,
`cargo +nightly fmt`, then the same clippy sequence -- passed.
ddidderr
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