Move reset, copy, and default policy resolution from
ZSTD_CCtxParams_init_internal into Rust through the existing repr(C)
parameter mirror. C retains validation and diagnostics, while focused tests
cover reset behavior, policy resolution, and compression-level handling.
Test Plan:
- capped nightly rustfmt check
- capped root clippy with all targets and -D warnings
- capped native make -j1
- capped upstream make -j1 -C tests test
- capped CLI clippy and 185 CLI tests
All integrated checks ran at the combined working-tree tip under a serial
40 GiB virtual-memory cap. Standalone root Rust unit linking remains
unavailable because the crate imports C-owned bridge symbols.
ddidderr
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