Move static CCtx construction sequencing out of the monolithic C leaf. Rust
now validates the existing public workspace contract, sequences workspace
creation, CCtx and auxiliary reservations, publication, and BMI2 setup, and
short-circuits callback-reported failures. C callbacks retain the private
cwksp and CCtx layouts plus the native workspace-size and CPU-feature rules.
Add matching ABI layout assertions and focused callback-order, reservation-
failure, workspace-capacity, and NULL-path unit coverage. Static-CDict and
heap-CCtx paths remain unchanged.
Test Plan:
- `rustfmt --edition 2021 --check rust/src/zstd_compress.rs` -- passed
- GCC and Clang `-fsyntax-only` checks on `zstd_compress.c` -- passed
- GCC syntax checks with `ZSTD_DISABLE_ASM=1` and
`ZSTD_ADDRESS_SANITIZER=1` -- passed
- `git diff --check` -- passed
- Cargo, make, native suites, and fuzzers not run per request
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