feat(embed): candle 임베딩 provider (NUMA-안전, opt-in) + v0.22.0

duo-socket NUMA 서버에서 fastembed(onnxruntime)가 intra-op 스레드를 48개로
하드코딩해 NUMA 힙 손상 → double-free 로 ingest 가 죽는 문제를 회피하기 위해,
같은 multilingual-e5-large 모델을 순수 Rust(candle)로 돌리는 opt-in 임베딩
provider 를 추가한다.

- 신규 crate kebab-embed-candle: CandleEmbedder (kebab_core::Embedder).
  hf-hub safetensors → XLMRobertaModel forward → mask mean-pool → L2 → e5
  prefix. candle 의존성 트리를 이 crate 에 격리 (core/config 외 kebab-* 의존 0).
- 스레드 캡: [models.embedding].num_threads + env KEBAB_EMBED_THREADS →
  글로벌 rayon 풀 1회 캡 (NUMA-안전 레버).
- kebab-app::embedder() 가 provider 분기 (fastembed/onnx/"" → 기존 경로 불변,
  candle → CandleEmbedder, 미지값 → 에러).
- Phase 0 스파이크 crate 제거 (production 흡수).
- 버전 0.21.1 → 0.22.0 (신규 config surface, pre-1.0 minor bump).

패리티: cosine_min=1.000000, max abs diff=2.01e-7 (< 1e-5) → embedding_version
유지, 재색인 0. fastembed default 동작/벡터 불변. wire schema 변경 없음.

검증(파일+exit code): clippy -D warnings EXIT=0(warning 0), test EXIT=0
(candle unit 5 + thread_cap rayon=4 + config 68), parity #[ignore] EXIT=0.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/kebab-config/src/lib.rs

@@ -155,11 +155,21 @@ impl NliCfg {
 
 #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
 pub struct EmbeddingModelCfg {
+    /// `fastembed` (default, onnxruntime) or `candle` (pure-Rust,
+    /// NUMA-safe). `none` disables embeddings (lexical-only). Unknown
+    /// values error at embedder construction.
     pub provider: String,
     pub model: String,
     pub version: String,
     pub dimensions: usize,
     pub batch_size: usize,
+    /// Cap on the CPU worker threads the `candle` provider spins up
+    /// (sizes the global rayon pool; env `KEBAB_EMBED_THREADS` overrides).
+    /// `0` = auto (rayon default = #cores). Lever to sidestep the
+    /// onnxruntime 48-thread NUMA double-free; ignored by the `fastembed`
+    /// provider. Defaulted on load so pre-0.22 config files still parse.
+    #[serde(default)]
+    pub num_threads: u32,
 }
 
 #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
@@ -707,6 +717,7 @@ impl Config {
                     version: "v1".to_string(),
                     dimensions: 1024,
                     batch_size: 64,
+                    num_threads: 0,
                 },
                 llm: LlmCfg {
                     provider: "ollama".to_string(),
@@ -964,6 +975,11 @@ impl Config {
                         self.models.embedding.batch_size = n;
                     }
                 }
+                "KEBAB_MODELS_EMBEDDING_NUM_THREADS" => {
+                    if let Ok(n) = v.parse::<u32>() {
+                        self.models.embedding.num_threads = n;
+                    }
+                }
 
                 // models.llm
                 "KEBAB_MODELS_LLM_PROVIDER" => self.models.llm.provider = v.clone(),