feat(embed): candle Metal (Apple Silicon GPU) opt-in build feature + v0.23.0

- kebab-embed-candle: `metal` feature → candle metal backend; select_device()
  picks Device::new_metal(0) (CPU fallback) under the feature, else Device::Cpu.
  .contiguous() before to_vec2 (Metal rejects strided views; CPU tolerates).
- feature passthrough: kebab-app/embed_metal → kebab-cli/embed_metal.
  Build on macOS: cargo build --release --features embed_metal.
- default (non-metal) path unchanged: clippy 0, candle units + thread_cap + parity pass.
- README + HOTFIXES: Mac-GPU-ingest → copy sqlite+lancedb → server CPU-query workflow.
- version 0.22.0 → 0.23.0 (opt-in build surface).

macOS-only compile; Metal execution/speed/parity validated by user on M4 Pro
(not buildable on the Linux CI/dev machine).

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

Cargo.toml

@@ -31,7 +31,7 @@ edition       = "2024"
 rust-version  = "1.85"
 license       = "MIT OR Apache-2.0"
 repository    = "https://github.com/altair823/kebab"
-version       = "0.22.0"   # v0.22.0 — candle 임베딩 provider (NUMA-안전, opt-in `provider=candle` + `num_threads`/KEBAB_EMBED_THREADS). fastembed default 불변, embedding_version 유지(재색인 0). — CLAUDE.md §Release 도그푸딩 트리거
+version       = "0.23.0"   # v0.23.0 — candle Metal(Apple Silicon GPU) opt-in build feature (`--features embed_metal`): M-series 맥에서 e5-large 임베딩을 GPU 로 → 대용량 ingest 가속. macOS 전용, 벡터는 CPU candle 과 호환. default(비-metal) 동작 불변. — CLAUDE.md §Release 도그푸딩 트리거
 
 # pre-v0.18 workspace-wide cleanup: enable clippy::pedantic group with
 # intentional allow-list. The allowed lints are either cosmetic (doc style),