feat: fix up #875 about MARGIN_WIDTH and FEATURE_SIZE (#880)

#875 にて `extern "C" fn generate_full_intermediate`の中で定義された二つ
の定数について、以下の変更を加える。

`MARGIN_WIDTH`: Rust APIのものを持って来るようにすることで重複を避ける
`FEATURE_SIZE`: 出力サイズのチェック時に失敗したとき、`ncols`が
                `FEATURE_SIZE`と異なるならパニックメッセージを別のものに
                する

Refs: #875

crates/voicevox_core/src/__internal/interop.rs

@@ -1,3 +1,6 @@
 pub mod raii;
 
-pub use crate::{metas::merge as merge_metas, synthesizer::blocking::PerformInference};
+pub use crate::{
+    metas::merge as merge_metas,
+    synthesizer::{blocking::PerformInference, MARGIN},
+};

crates/voicevox_core/src/synthesizer.rs

@@ -1,5 +1,7 @@
 use crate::asyncs::{Async, BlockingThreadPool, SingleTasked};
 
+pub use self::inner::MARGIN;
+
 /// [`blocking::Synthesizer::synthesis`]および[`nonblocking::Synthesizer::synthesis`]のオプション。
 ///
 /// [`blocking::Synthesizer::synthesis`]: blocking::Synthesizer::synthesis
@@ -132,7 +134,7 @@ mod inner {
     const PADDING_FRAME_LENGTH: usize = 38; // (0.4秒 * 24000Hz / 256.0).round()
     /// 音声生成の際、音声特徴量の前後に確保すべきマージン幅（フレーム数）
     /// モデルの受容野から計算される
-    const MARGIN: usize = 14;
+    pub const MARGIN: usize = 14;
     /// 指定した音声区間に対応する特徴量を両端にマージンを追加した上で切り出す
     fn crop_with_margin(audio: &AudioFeature, range: Range<usize>) -> ndarray::ArrayView2<'_, f32> {
         if range.start > audio.frame_length || range.end > audio.frame_length {

crates/voicevox_core_c_api/src/compatible_engine.rs

@@ -374,15 +374,15 @@ pub unsafe extern "C" fn generate_full_intermediate(
     speaker_id: *mut i64,
     output: *mut f32,
 ) -> bool {
+    use voicevox_core::__internal::interop::MARGIN as MARGIN_WIDTH;
+    const FEATURE_SIZE: usize = 80;
     init_logger_once();
     assert_aligned(f0);
     assert_aligned(phoneme);
     assert_aligned(speaker_id);
     assert_aligned(output);
     let length = length as usize;
     let phoneme_size = phoneme_size as usize;
-    const MARGIN_WIDTH: usize = 14;
-    const FEATURE_SIZE: usize = 80;
     let synthesizer = &*lock_synthesizer();
     let result = ensure_initialized!(synthesizer).generate_full_intermediate(
         length,
@@ -396,7 +396,11 @@ pub unsafe extern "C" fn generate_full_intermediate(
         Ok(output_arr) => {
             let output_len = (length + 2 * MARGIN_WIDTH) * FEATURE_SIZE;
             if output_arr.len() != output_len {
-                panic!("expected {}, got {}", output_len, output_arr.len());
+                if output_arr.ncols() != FEATURE_SIZE {
+                    panic!("the feature size is expected to be {FEATURE_SIZE}");
+                } else {
+                    panic!("expected {}, got {}", output_len, output_arr.len());
+                }
             }
             let output_arr = output_arr.as_standard_layout();
             // SAFETY: The safety contract must be upheld by the caller.