Move the mutable external raw-sequence-store assignment and cursor reset
behind a Rust-owned pointer projection. Keep the public C entry point and
its stage/LDM assertions in C, without exposing RawSeqStore_t to Rust.
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 reference_external_sequences unit tests
ddidderr
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