feat(ldm): move sequence-store emission into Rust

Remove the LDM-specific C sequence-store callback and reuse the existing Rust
sequence-store leaf from the fast matcher through a crate-local opaque wrapper.
The LDM C shim now retains only private match-state extraction and compressor
dispatch, with explicit SeqDef and SeqStore_t layout checks protecting the
cross-module pointer contract. The Rust boundary documentation now reflects
that sequence-store encoding is Rust-owned.

Test Plan:
- ulimit -v 41943040 && cargo fmt --manifest-path rust/Cargo.toml && git diff --check
- ulimit -v 41943040 && CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml zstd_ldm::tests --lib (7 passed)
- ulimit -v 41943040 && CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml store_seq_preserves_literal --lib (1 passed)
- ulimit -v 41943040 && CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml --lib (683 passed)
- ulimit -v 41943040 && CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings
- ulimit -v 41943040 && make -B -C programs -j1 zstd (passed; existing fileio const-cast warnings)
- ulimit -v 41943040 && make -C tests -j1 test-zstream ZSTREAM_TESTTIME=-T1s (84 tests and both short fuzz rounds passed; existing zstreamtest warning)

rust/src/zstd_fast.rs

@@ -293,6 +293,28 @@ unsafe fn store_seq(
     seq_store.sequences = sequence.wrapping_add(1);
 }
 
+/// Reuse the sequence-store leaf from another Rust compression module without
+/// exposing this module's C-layout projection in its public API.
+pub(crate) unsafe fn store_seq_opaque(
+    seq_store: *mut c_void,
+    lit_length: usize,
+    literals: *const c_void,
+    lit_limit: *const c_void,
+    off_base: u32,
+    match_length: usize,
+) {
+    unsafe {
+        store_seq(
+            seq_store.cast::<SeqStore_t>(),
+            lit_length,
+            literals.cast::<u8>(),
+            lit_limit.cast::<u8>(),
+            off_base,
+            match_length,
+        )
+    }
+}
+
 #[inline]
 unsafe fn match4_found(
     current: *const u8,
@@ -1576,11 +1598,11 @@ mod tests {
         };
 
         unsafe {
-            store_seq(
-                &mut store,
+            store_seq_opaque(
+                (&mut store as *mut SeqStore_t).cast(),
                 source.len(),
-                source.as_ptr(),
-                source.as_ptr().add(source.len()),
+                source.as_ptr().cast(),
+                source.as_ptr().add(source.len()).cast(),
                 13,
                 3 + 0x1_0000,
             );

rust/src/zstd_ldm.rs

@@ -12,6 +12,7 @@
 use crate::errors::{ERR_isError, ZstdErrorCode, ERROR};
 use crate::mem::{MEM_64bits, MEM_isLittleEndian, MEM_read16, MEM_read32, MEM_readST};
 use crate::xxhash::XXH64;
+use crate::zstd_fast::store_seq_opaque;
 use std::ffi::c_void;
 use std::mem::size_of;
 use std::os::raw::c_int;
@@ -98,14 +99,6 @@ unsafe extern "C" {
         src: *const c_void,
         src_size: usize,
     ) -> usize;
-    fn ZSTD_ldm_rust_storeSeq(
-        seq_store: *mut c_void,
-        lit_length: usize,
-        literals: *const c_void,
-        lit_limit: *const c_void,
-        off_base: u32,
-        match_length: usize,
-    );
     fn ZSTD_ldm_rust_setLdmSeqStore(context: *mut c_void, raw_seq_store: *const c_void);
 }
 
@@ -990,11 +983,11 @@ pub unsafe extern "C" fn ZSTD_rust_ldm_blockCompress(
             *reps.add(2) = *reps.add(1);
             *reps.add(1) = *reps;
             *reps = sequence.offset;
-            ZSTD_ldm_rust_storeSeq(
+            store_seq_opaque(
                 seq_store,
                 new_lit_length,
-                input_position.wrapping_sub(new_lit_length).cast::<c_void>(),
-                input_end.cast::<c_void>(),
+                input_position.wrapping_sub(new_lit_length).cast(),
+                input_end.cast(),
                 sequence.offset.wrapping_add(ZSTD_REP_NUM as u32),
                 sequence.match_length as usize,
             );