feat(rust): migrate high-level runtime paths

Move long-distance matching and high-level decompression from C shims into
Rust. The decoder now owns context, dictionary, parameter, one-shot, and
buffered streaming state while C retains allocation/configuration, legacy,
and trace leaves.

Move CLI parsing, safety policy, and dispatch into a separate Rust static
archive. Keeping it separate prevents library builds from retaining FIO
symbols, while C continues to own file opening, replacement, and I/O.
Program targets now select matching compression/decompression archives.

The remaining C boundary is intentional: high-level compression, optimal
parsing, dictionary building, legacy callbacks, and CLI file I/O still need
migration.

Test Plan:
- cargo test --all-targets (native and i686)
- cargo test --all-targets in rust/cli (native and i686)
- CLI crate compression-only and decompression-only feature tests
- native and i686 fuzzer/zstreamtest runs, plus legacy and dictionary tests
- ZSTD_C_PREDICT and ZSTD_HEAPMODE=0 fuzzer coverage
- library, dynamic-link, and program-target build/round-trip matrix

Refs: rust/README.md

rust/README.md

@@ -37,6 +37,8 @@ zstd ABI:
     including row-based and dictionary search variants.
   - `zstd_opt_tree` maintains the binary-tree index used by optimal matching;
     the dynamic-programming optimal parser itself remains in C for now.
+  - `zstd_ldm` implements long-distance-match parameter selection, table
+    maintenance, sequence generation, and sequence consumption.
 - Runtime support
   - `threading` provides platform pthread wrappers required by zstd headers.
   - `pool` implements the bounded worker pool used by multithreaded compression.
@@ -45,11 +47,20 @@ zstd ABI:
 - Block decompression
   - `zstd_decompress_block` decodes literal and sequence sections, maintains
     FSE/Huffman repeat state, and executes compressed-block sequences.
+  - `zstd_decompress` owns the public decompression context, one-shot,
+    dictionary, parameter, and streaming state machines.  Its C shim retains
+    configuration-dependent context allocation plus legacy and trace leaves.
+- Command-line frontend
+  - `zstd_cli` owns the Rust parser, safety policy, and dispatch.  It is built
+    by the separate `cli/` static-library package only for program archives,
+    so library builds do not acquire program-only dependencies.  The C
+    `fileio` backend still owns file opening, safe replacement, sparse writes,
+    metadata, and streaming I/O.
 
-The optimal block matcher, high-level frame decompression, dictionary-building,
-legacy, and CLI translation units are still C. They must move before the
-rewrite is complete. Keeping that boundary explicit prevents a passing hybrid
-build from being mistaken for the final all-Rust result.
+The optimal block matcher, high-level frame compression, dictionary-building,
+legacy decoding callbacks, and the CLI file-I/O backend are still C. They must
+move before the rewrite is complete. Keeping that boundary explicit prevents a
+passing hybrid build from being mistaken for the final all-Rust result.
 
 ## Compatibility boundary
 
@@ -79,6 +90,16 @@ cargo test --all-targets
 cargo build --release
 ```
 
+The program-only Rust archive has its own feature matrix and should be checked
+from `rust/cli` as well:
+
+```sh
+cargo clippy --all-targets -- -D warnings
+cargo test --all-targets
+cargo test --no-default-features --features compression --all-targets
+cargo test --no-default-features --features decompression --all-targets
+```
+
 Then run original compatibility tests from the repository root, starting with
 the narrow target for the component being migrated. For example: