feat(rust): port frame serialization

Continue the incremental zstd_compress.c migration with its frame
serialization leaves: ZSTD_writeFrameHeader(), the public
ZSTD_writeSkippableFrame() ABI, and ZSTD_writeLastEmptyBlock() now live
in rust/src/zstd_compress_frame.rs.

ZSTD_writeFrameHeader() reads five fields out of ZSTD_CCtx_params, whose
layout is private to zstd_compress.c and sensitive to build
configuration.  Rather than mirror that structure in Rust, the C
function keeps its original static signature and forwards the scalar
fields to ZSTD_rust_writeFrameHeader(), so no parameter-structure layout
crosses the language boundary.  The other two functions take only
pointer/size arguments and are exported directly, replacing their C
bodies outright.

Behavior differences are limited to hardening in release builds: the
Rust leaf clamps a window-size shift that C would leave undefined for a
malformed windowLog, and ZSTD_writeSkippableFrame() rejects a srcSize +
header overflow instead of comparing against a wrapped sum.  Both paths
are unreachable through validated callers, so compressed output is
byte-identical.

Remaining zstd_compress.c work: parameter selection and validation,
context lifecycle, block dispatch, dictionary loading, and the streaming
state machine.

Test plan:
- cd rust && cargo fmt --check && cargo clippy --all-targets -- -D
  warnings && cargo test --all-targets (118 tests, includes new frame
  header/skippable/last-block reference vectors)
- make -C tests fuzzer && ./tests/fuzzer -i1 --no-big-tests
- make -C tests test-rust-lib-smoke

rust/src/lib.rs

@@ -24,6 +24,8 @@ pub mod zstd_common;
 #[cfg(feature = "compression")]
 pub mod zstd_compress_api;
 #[cfg(feature = "compression")]
+pub mod zstd_compress_frame;
+#[cfg(feature = "compression")]
 pub mod zstd_compress_literals;
 #[cfg(feature = "compression")]
 pub mod zstd_compress_sequences;