feat(compress): move CCtx workspace estimator into Rust

The CCtx workspace estimator still assembled its private sizing formula in C,
even though the match-state and component-sum leaves already lived in Rust.
That left buffering, LDM, static-context, and external-sequence sizing policy
outside the rewrite boundary.

Move the complete size_t formula into a Rust entry point. C now supplies only
private object sizes, sanitizer redzone policy, and LDM scalar fields through a
small ABI record. Rust retains the C wrapping/alignment behavior and focused
branch coverage while the C adapter preserves the existing public estimator
surface.

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 estimate_cctx_workspace_size
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml --release (667 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

@@ -165,9 +165,10 @@ parameter validation and init-then-begin ordering now run in Rust.
 CDict advanced private workspace construction, private static-CCtx and
 static-CDict workspace construction and dictionary-content allocation/loading,
 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,
+advanced-CDict custom-memory validation, workspace-size query/allocation,
+allocation/create/init cleanup ordering, and the CCtx workspace-size formula;
+C retains private layout-size inputs, workspace 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