ZSTD_compress_advanced_internal() still encoded the one-shot begin-then-end
sequence in C after public advanced parameter validation had moved behind Rust.
That left the public compression boundary split between Rust policy and a C
orchestration body, and made the begin-error ordering implicit in the C path.
Add an explicit C-layout state with opaque begin and end callbacks. Rust now
owns the ordering, forwards the source size as the pledged size, propagates a
begin error before invoking the end callback, and returns the end result. The
callbacks retain the private ZSTD_CCtx and ZSTD_CCtx_params layouts, so the
change moves policy and sequencing without exposing window, workspace,
matchfinder, or context internals to Rust. The existing advanced and
using-dictionary wrappers continue to use the same C private operations.
Test Plan:
- `git diff --cached --check` -- passed.
- `rustfmt --check --edition 2021 rust/src/zstd_compress.rs` -- passed.
- Not run: Cargo, make, native tests, builds, or heavy verification per request.
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