refactor(compress): expose LDM skip leaves from Rust

Keep the existing internal ZSTD_ldm_* ABI names while making the two
sequence-store skip operations direct Rust exports. Remove only the C
forwarding declarations and bodies; LDM state projection, configuration
constants, and block-compressor adapters remain in C.

Test Plan:
- clang -fsyntax-only on lib/compress/zstd_ldm.c
- git diff --check
- Full capped native/upstream suite pending after this commit

rust/src/zstd_ldm.rs

@@ -903,7 +903,7 @@ unsafe fn skip_sequences(raw_seq_store: *mut RawSeqStore, mut src_size: usize, m
 }
 
 #[no_mangle]
-pub unsafe extern "C" fn ZSTD_rust_ldm_skipSequences(
+pub unsafe extern "C" fn ZSTD_ldm_skipSequences(
     raw_seq_store: *mut c_void,
     src_size: usize,
     min_match: u32,
@@ -912,7 +912,7 @@ pub unsafe extern "C" fn ZSTD_rust_ldm_skipSequences(
 }
 
 #[no_mangle]
-pub unsafe extern "C" fn ZSTD_rust_ldm_skipRawSeqStoreBytes(
+pub unsafe extern "C" fn ZSTD_ldm_skipRawSeqStoreBytes(
     raw_seq_store: *mut c_void,
     nb_bytes: usize,
 ) {
@@ -978,7 +978,7 @@ pub unsafe extern "C" fn ZSTD_rust_ldm_blockCompress(
         let last_literals = unsafe {
             ZSTD_ldm_rust_compressLiterals(block_context, seq_store, reps, src, src_size)
         };
-        unsafe { ZSTD_rust_ldm_skipRawSeqStoreBytes(raw_seq_store.cast::<c_void>(), src_size) };
+        unsafe { ZSTD_ldm_skipRawSeqStoreBytes(raw_seq_store.cast::<c_void>(), src_size) };
         return last_literals;
     }