Move the frozen v0.7 decoder, dictionary handling, bufferless streaming,
and buffered streaming implementation into the Rust legacy module. Keep
the historical C translation unit as a declaration-only ABI shim so the
existing C callers and build selection remain unchanged.
The port preserves the v0.7 entropy and frame boundaries, while fixing the
Rust-side literal-tail bookkeeping and making buffered header loading revisit
the complete frame header before sizing its rolling buffers.
Test Plan:
- cargo test --manifest-path rust/Cargo.toml --no-default-features --features decompression,legacy-v07 legacy::zstd_v07::tests
- cargo clippy --manifest-path rust/Cargo.toml -- -D warnings
- make -B -C programs zstd V=1
- decode a v0.7.5 fixture and compare its 200000-byte output
- make -C tests check V=1
ddidderr
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