feat(rust): port superblock compression

Move subblock partitioning, literal and sequence section emission, entropy
fallback, and repcode repair into Rust. Keep a narrow C wrapper to read the
opaque compression context and invoke the existing C entropy-stat builder;
the Rust side owns only verified C-shaped leaf layouts.

Test Plan:
- cargo clippy
- cargo clippy --benches
- cargo clippy --tests
- cargo +nightly fmt
- cargo test zstd_compress_superblock::tests -- --nocapture
- cargo test --target i686-unknown-linux-gnu zstd_compress_superblock::tests
- byte-exact C control across 16 target-size and input-shape cases
- fuzzer, zstreamtest, invalidDictionaries, and bounded native test matrix

Refs: Rust sequence entropy port 887af204

rust/src/lib.rs

@@ -25,6 +25,8 @@ pub mod zstd_common;
 pub mod zstd_compress_literals;
 #[cfg(feature = "compression")]
 pub mod zstd_compress_sequences;
+#[cfg(feature = "compression")]
+pub mod zstd_compress_superblock;
 #[cfg(feature = "decompression")]
 pub mod zstd_ddict;
 #[cfg(feature = "compression")]