fix(dict-builder): stabilize optimized dictionary scoring

Optimized dictionary scoring used the requested dictionary ID while building
its scoring CDict, so --dictID could change candidate quality and output
content. Parallel FastCover could also choose different equal-score candidates
based on completion order. Clear the ID only while constructing the scoring
CDict, restore it immediately afterward, and use deterministic (d, k)
tie-breakers in Cover and FastCover. The requested ID now remains output
metadata instead of influencing dictionary content or repeat-run stability.

Test Plan:
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml scoring_dictionary_id_is_restored_after_the_score --lib
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings
- ulimit -v 41943040; make -j1
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml --all-targets
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/cli/Cargo.toml --all-targets
- ulimit -v 41943040; make -j1 -C tests test

rust/src/dict_builder_cover.rs

@@ -541,7 +541,10 @@ fn best_finish_slot(
         .lock()
         .unwrap_or_else(|poison| poison.into_inner());
     data.live_jobs = data.live_jobs.wrapping_sub(1);
-    if selection.totalCompressedSize < data.compressed_size {
+    let improves_score = selection.totalCompressedSize < data.compressed_size;
+    let wins_equal_score = selection.totalCompressedSize == data.compressed_size
+        && (parameters.d, parameters.k) < (data.parameters.d, data.parameters.k);
+    if improves_score || wins_equal_score {
         if data.dict.is_null() || data.dict_capacity < selection.dictSize {
             let replacement = unsafe { malloc_bytes(selection.dictSize) };
             if replacement.is_null() && selection.dictSize != 0 {
@@ -629,6 +632,22 @@ unsafe extern "C" {
     ) -> usize;
 }
 
+unsafe fn with_dictionary_id_cleared<T>(
+    dict: *mut u8,
+    dict_buffer_capacity: usize,
+    action: impl FnOnce() -> T,
+) -> T {
+    if dict_buffer_capacity < 8 {
+        return action();
+    }
+    let dict_id = unsafe { dict.add(4).cast::<u32>() };
+    let saved_id = unsafe { ptr::read_unaligned(dict_id) };
+    unsafe { ptr::write_unaligned(dict_id, 0) };
+    let result = action();
+    unsafe { ptr::write_unaligned(dict_id, saved_id) };
+    result
+}
+
 #[no_mangle]
 pub unsafe extern "C" fn COVER_sum(samples_sizes: *const usize, nb_samples: c_uint) -> usize {
     if nb_samples == 0 {
@@ -781,12 +800,19 @@ pub unsafe extern "C" fn COVER_checkTotalCompressedSize(
     }
     let dst = unsafe { malloc_bytes(dst_capacity) };
     let cctx = unsafe { ZSTD_createCCtx() };
+    /* The dictionary ID is output metadata, not dictionary content.  The
+     * fixed-frame `ZSTD_compress_usingCDict()` score includes that metadata,
+     * so temporarily clear it while constructing the scoring CDict.  This
+     * keeps `--dictID` from changing which candidate wins optimization while
+     * leaving the requested ID in the finalized dictionary. */
     let cdict = unsafe {
-        ZSTD_createCDict(
-            dict.cast(),
-            dict_buffer_capacity,
-            parameters.zParams.compressionLevel,
-        )
+        with_dictionary_id_cleared(dict, dict_buffer_capacity, || {
+            ZSTD_createCDict(
+                dict.cast(),
+                dict_buffer_capacity,
+                parameters.zParams.compressionLevel,
+            )
+        })
     };
     if dst.is_null() || cctx.is_null() || cdict.is_null() {
         if !cctx.is_null() {
@@ -1287,6 +1313,22 @@ mod tests {
         );
     }
 
+    #[test]
+    fn scoring_dictionary_id_is_restored_after_the_score() {
+        let mut dictionary = [0u8; 16];
+        dictionary[4..8].copy_from_slice(&0x1234_5678u32.to_ne_bytes());
+        let observed_id = unsafe {
+            with_dictionary_id_cleared(dictionary.as_mut_ptr(), dictionary.len(), || {
+                ptr::read_unaligned(dictionary.as_ptr().add(4).cast::<u32>())
+            })
+        };
+        assert_eq!(observed_id, 0);
+        assert_eq!(
+            u32::from_ne_bytes(dictionary[4..8].try_into().unwrap()),
+            0x1234_5678
+        );
+    }
+
     #[test]
     fn context_groups_repeated_dmers_by_sample() {
         let samples = [b'a'; 45];

rust/src/dict_builder_fastcover.rs

@@ -524,7 +524,10 @@ impl FastBest {
             .lock()
             .unwrap_or_else(|poison| poison.into_inner());
         state.live_jobs = state.live_jobs.wrapping_sub(1);
-        if selection.totalCompressedSize < state.compressed_size {
+        let improves_score = selection.totalCompressedSize < state.compressed_size;
+        let wins_equal_score = selection.totalCompressedSize == state.compressed_size
+            && (parameters.d, parameters.k) < (state.parameters.d, state.parameters.k);
+        if improves_score || wins_equal_score {
             if state.dict.is_null() || state.dict_size < selection.dictSize {
                 unsafe { free_bytes(state.dict) };
                 let replacement = unsafe { malloc_bytes(selection.dictSize) };