Move ZSTD_freeCCtx null handling, static-context rejection, content/object
free ordering, and embedded-workspace policy behind a Rust-owned ABI bridge.
C retains dictionary, multithread, workspace, and custom allocator teardown
through callbacks, and ZSTD_freeCStream continues to use the CCtx path.
Test Plan:
- cargo test --manifest-path rust/Cargo.toml --lib
- cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings
- make -B -C programs -j1 zstd
- make -C tests -j1 test-zstream ZSTREAM_TESTTIME=-T1s
- focused free_cctx unit tests
ddidderr
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