Keep the opaque ZSTD_CCtx_params copy operation in C, but make Rust own the public advanced2 null-source validation and copy-before-preparation ordering. The bridge now rejects a missing source before any allocator or workspace callback, with explicit ABI assertions and focused order/null-input tests.
Test Plan: Not run in this atomic commit; the capped serial Rust, native, smoke, and original test-suite verification follows.
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