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/README.md

@@ -44,9 +44,10 @@ zstd ABI:
     and the outer block-dispatch decision. `zstd_compress` also owns ordinary
     sequence-block entropy emission, sequence collection, the legacy RLE
     compatibility gate, and sequence-store construction; C supplies only the
-    private matchfinder, LDM, external-sequence-producer, and state-preparation
-    callbacks. Rust also owns the frame-chunk block loop, including block
-    sizing, target/split/internal dispatch, output accounting, and frame-state
+    private matchfinder, LDM block-preparation/consumption, external-sequence-
+    producer, and state-preparation callbacks. Rust also owns the frame-chunk
+    block loop, including block sizing, target/split/internal dispatch, output
+    accounting, and frame-state
     updates; C supplies the private block-compression callbacks. Rust also
     owns the single-threaded buffered/stable stream state machine, including
     direct versus buffered output, pending-output draining, and frame reset
@@ -78,7 +79,8 @@ zstd ABI:
     `zstd_opt` owns the dynamic-programming price model, optimal parse, and
     sequence emission.
   - `zstd_ldm` implements long-distance-match parameter selection, table
-    maintenance, sequence generation, and sequence consumption.
+    maintenance, sequence generation, sequence consumption, and sequence-store
+    encoding.
 - Dictionary building
   - `divsufsort` constructs the suffix array that drives the legacy `ZDICT`
     trainer (`ZDICT_trainFromBuffer_legacy`); `dict_builder_zdict`,