refactor(ldm): expose max sequence bound from Rust

The LDM sequence-space helper was already implemented in Rust, but C still
owned the public symbol and forwarded through a pointer-based Rust bridge.
Make the Rust parameter representation public and ABI-compatible for the
original by-value `ldmParams_t` call, then have Rust compare `enableLdm`
directly with `ZSTD_ps_enable` before applying the sequence bound. Remove only
the redundant C bridge declaration and wrapper; table sizing, state, and
sequence generation remain unchanged.

Test Plan:
- `git diff --check` -- passed.
- `git diff --cached --check` -- passed.
- `cc -std=c99 -Ilib -Ilib/common -Ilib/compress -Ilib/decompress -Ilib/dict -fsyntax-only lib/compress/zstd_ldm.c` -- passed.
- `cargo +nightly fmt --manifest-path rust/Cargo.toml --all -- --check` -- passed.
- Cargo build/test, make, and other heavy checks were intentionally not run per the requested lightweight-only scope.

lib/compress/zstd_ldm.c

@@ -256,8 +256,6 @@ void ZSTD_rust_ldm_adjustParameters(
         U32 hashLogMax, U32 bucketSizeLogMax, int btultra);
 size_t ZSTD_rust_ldm_getTableSize(
         const void* params, int enableLdm, size_t redzoneSize);
-size_t ZSTD_rust_ldm_getMaxNbSeq(
-        const void* params, int enableLdm, size_t maxChunkSize);
 void ZSTD_rust_ldm_fillHashTable(
         void* hashTable, BYTE* bucketOffsets, const BYTE* base,
         const BYTE* ip, const BYTE* iend, const void* params);
@@ -357,12 +355,6 @@ size_t ZSTD_ldm_getTableSize(ldmParams_t params)
             &params, params.enableLdm == ZSTD_ps_enable, redzoneSize);
 }
 
-size_t ZSTD_ldm_getMaxNbSeq(ldmParams_t params, size_t maxChunkSize)
-{
-    return ZSTD_rust_ldm_getMaxNbSeq(
-            &params, params.enableLdm == ZSTD_ps_enable, maxChunkSize);
-}
-
 void ZSTD_ldm_fillHashTable(
         ldmState_t* ldmState, const BYTE* ip,
         const BYTE* iend, ldmParams_t const* params)

rust/src/zstd_ldm.rs

@@ -27,6 +27,7 @@ const ZSTD_FAST_STRATEGY: c_int = 1;
 const ZSTD_DFAST_STRATEGY: c_int = 2;
 const ZSTD_STRATEGY_MAX: c_int = 9;
 const ZSTD_BTOPT: c_int = 7;
+const ZSTD_PS_ENABLE: c_int = 1;
 
 const LDM_FAST_TABLES_NONE: c_int = 0;
 const LDM_FAST_TABLES_FAST: c_int = 1;
@@ -58,7 +59,7 @@ struct RawSeqStore {
 }
 
 #[repr(C)]
-struct LdmParams {
+pub struct LdmParams {
     enable_ldm: c_int,
     hash_log: u32,
     bucket_size_log: u32,
@@ -715,13 +716,8 @@ pub unsafe extern "C" fn ZSTD_rust_ldm_getTableSize(
 }
 
 #[no_mangle]
-pub unsafe extern "C" fn ZSTD_rust_ldm_getMaxNbSeq(
-    params: *const c_void,
-    enable_ldm: c_int,
-    max_chunk_size: usize,
-) -> usize {
-    let params = unsafe { &*params.cast::<LdmParams>() };
-    if enable_ldm != 0 {
+pub extern "C" fn ZSTD_ldm_getMaxNbSeq(params: LdmParams, max_chunk_size: usize) -> usize {
+    if params.enable_ldm == ZSTD_PS_ENABLE {
         max_chunk_size / params.min_match_length as usize
     } else {
         0
@@ -1090,6 +1086,29 @@ mod tests {
         assert_eq!(params.min_match_length, 64);
     }
 
+    #[test]
+    fn max_nb_seq_only_applies_when_ldm_is_enabled() {
+        let params = LdmParams {
+            enable_ldm: ZSTD_PS_ENABLE,
+            hash_log: 0,
+            bucket_size_log: 0,
+            min_match_length: 64,
+            hash_rate_log: 0,
+            window_log: 0,
+        };
+        assert_eq!(ZSTD_ldm_getMaxNbSeq(params, 640), 10);
+
+        let params = LdmParams {
+            enable_ldm: 0,
+            hash_log: 0,
+            bucket_size_log: 0,
+            min_match_length: 64,
+            hash_rate_log: 0,
+            window_log: 0,
+        };
+        assert_eq!(ZSTD_ldm_getMaxNbSeq(params, 640), 0);
+    }
+
     #[test]
     fn optimal_parser_starts_at_btopt_strategy() {
         assert!(!use_optimal_parser(ZSTD_BTOPT - 1));