feat(compress): port multithreaded resource pools

Move the reusable buffer and compression-context pools behind a Rust
implementation with a narrow C ABI adapter. The scheduler, job table, serial
LDM state, and stream orchestration remain in C until their private layouts
are ported.

Keep context types opaque across Rust modules so the pool bridge does not
depend on private C layout declarations. This also keeps the existing custom
allocator and pool replacement contracts intact.

Test Plan:
- RUSTC_WRAPPER= CARGO_BUILD_RUSTC_WRAPPER= cargo clippy --manifest-path
  rust/Cargo.toml --all-targets -- -D warnings
- RUSTC_WRAPPER= CARGO_BUILD_RUSTC_WRAPPER= cargo test --manifest-path
  rust/Cargo.toml zstdmt_compress
- make -B -C lib lib-mt
- git diff --cached --check

rust/src/dict_builder_cover.rs

@@ -596,15 +596,8 @@ fn best_snapshot(slot: &BestSlot) -> BestSnapshot {
     }
 }
 
-#[repr(C)]
-struct ZSTD_CCtx {
-    _private: [u8; 0],
-}
-
-#[repr(C)]
-struct ZSTD_CDict {
-    _private: [u8; 0],
-}
+type ZSTD_CCtx = c_void;
+type ZSTD_CDict = c_void;
 
 unsafe extern "C" {
     fn ZDICT_finalizeDictionary(