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>

crates/kebab-app/Cargo.toml

@@ -100,6 +100,8 @@ reqwest      = { version = "0.12", default-features = false, features = ["blocki
 # disable path 없음; 이 feature 는 spec §6.3 명시를 honor 하는 role 만.
 default = ["fts_korean_morphological"]
 fts_korean_morphological = []
+# opt-in (macOS): candle embedder runs on the Apple Silicon GPU. See kebab-embed-candle.
+embed_metal = ["kebab-embed-candle/metal"]
 
 [lints]
 workspace = true