feat(compress): move literal compression policy to Rust

ZSTD_literalsCompressionIsDisabled() previously switched on the private
ZSTD_CCtx_params layout in a header inline helper. Replace that switch with a
scalar ABI call that extracts only literalCompressionMode, strategy, and
targetLength in C. The Rust helper preserves the enable and disable results,
the fast-strategy auto rule, and C's debug assertion plus release fallthrough
for invalid modes. Keep callers and compression state ownership in C, and
cover the policy truth table and debug invalid-mode invariant in Rust.

Test Plan:
- Compression-feature clippy for the library, benches, and tests before and
  after nightly formatting -- passed.
- `cargo +nightly fmt --manifest-path rust/Cargo.toml` -- passed.
- `cargo test --manifest-path rust/Cargo.toml --no-default-features
  --features compression zstd_compress_params` -- passed (43 tests).
- `make -B -C lib -j2 lib` -- passed.
- `make -C tests test-rust-lib-smoke` -- passed.
- `tests/fuzzer -s4560 -t47 -i48 -v` and `-s4560 -t56 -i57 -v` -- passed.
- `make -C tests -j2 test-zstream` -- passed (84 named, 5,416, and 8,387
  randomized cases).
- `git diff --check` and `git diff --cached --check` -- passed.

lib/compress/zstd_compress_internal.h

@@ -668,19 +668,16 @@ MEM_STATIC size_t ZSTD_minGain(size_t srcSize, ZSTD_strategy strat)
     return (srcSize >> minlog) + 2;
 }
 
+int ZSTD_rust_params_literalsCompressionIsDisabled(int literalCompressionMode,
+                                                   int strategy,
+                                                   U32 targetLength);
+
 MEM_STATIC int ZSTD_literalsCompressionIsDisabled(const ZSTD_CCtx_params* cctxParams)
 {
-    switch (cctxParams->literalCompressionMode) {
-    case ZSTD_ps_enable:
-        return 0;
-    case ZSTD_ps_disable:
-        return 1;
-    default:
-        assert(0 /* impossible: pre-validated */);
-        ZSTD_FALLTHROUGH;
-    case ZSTD_ps_auto:
-        return (cctxParams->cParams.strategy == ZSTD_fast) && (cctxParams->cParams.targetLength > 0);
-    }
+    return ZSTD_rust_params_literalsCompressionIsDisabled(
+            (int)cctxParams->literalCompressionMode,
+            (int)cctxParams->cParams.strategy,
+            cctxParams->cParams.targetLength);
 }
 
 /*! ZSTD_safecopyLiterals() :