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/README.md

@@ -31,6 +31,9 @@ zstd ABI:
   - `zstd_presplit` chooses split points for full compression blocks.
   - `zstd_compress_literals` emits raw, RLE, and Huffman literal sections
     while preserving the compressor's Huffman-table repeat state.
+  - `zstd_compress_frame` serializes frame headers, skippable frames, and the
+    last empty block; it takes scalar frame parameters so the C-owned
+    `ZSTD_CCtx_params` layout never crosses the language boundary.
   - `zstd_fast` and `zstd_double_fast` implement the single- and two-table
     fast block match finders, including attached and external dictionary paths.
   - `zstd_lazy` implements greedy, lazy, lazy2, and binary-tree matching,