feat(compress): move advanced CDict allocation policy into Rust

The advanced CDict bridge previously left custom-memory validation,
workspace allocation, and create/init failure ordering inside one C helper.
That made the Rust policy seam stop before allocation and obscured which
allocation owned cleanup after private CDict construction failed.

Move validation, workspace-size query dispatch, allocation, and cleanup
ordering into the Rust bridge. C retains the private workspace-size formula,
workspace/object placement, allocator callbacks, and CDict initialization.
The focused probes cover invalid allocators, allocation failure, creation
failure, and initialization failure with exact callback ordering.

Test Plan:
- cargo fmt --manifest-path rust/Cargo.toml -- --check
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml --release create_cdict_advanced
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml --release (665 passed)
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --release --all-targets -- -D warnings
- ulimit -v 41943040; make -B -C programs -j1 zstd
- ulimit -v 41943040; make -C tests -j1 test-zstream ZSTREAM_TESTTIME=-T1s (84 tests and both short fuzzer rounds)

rust/README.md

@@ -162,10 +162,12 @@ sequencing, legacy public CDict-begin frame-policy construction and
 unknown-source pledge, and the compressBegin_usingDict family’s unknown-source
 parameter selection and default-level normalization, and public advanced-begin
 parameter validation and init-then-begin ordering now run in Rust.
-CDict advanced allocation/lifecycle machinery, private static-CCtx and
+CDict advanced private workspace construction, private static-CCtx and
 static-CDict workspace construction and dictionary-content allocation/loading,
-and advanced-CDict private workspace construction and dictionary-content
-loading remain in C. Reset policy,
+and advanced-CDict dictionary-content loading remain in C. Rust now owns
+advanced-CDict custom-memory validation, workspace-size query/allocation, and
+allocation/create/init cleanup ordering; C retains the private workspace
+size formula, layout, and allocator callbacks. Reset policy,
 private CCtx/matchfinder/workspace operations, and codec/adaptive-policy
 callbacks remain in C. CDict initialization ordering and scalar publication,
 shared compression-begin dictionary selection, CDict reset attach-versus-copy