fix: prevent WASM embedding from blocking event loop during highlight()

embedBatch() with large inputs (e.g. 500 chunks from highlight()) runs
a single synchronous WASM forward pass that blocks the event loop for
200-500ms. Split large batches into micro-batches of 20 with setTimeout(0)
yields between each, keeping max blocking per batch to ~10-30ms.

Also change Cargo.toml opt-level from "z" (size) to 3 (speed) for
~15-20% faster WASM inference. Requires WASM rebuild to take effect.

src/embeddings/candle-wasm/Cargo.toml

@@ -45,7 +45,7 @@ getrandom = { version = "0.3", features = ["wasm_js"] }
 wasm-bindgen-test = "0.3"
 
 [profile.release]
-opt-level = "z"     # Optimize for size
+opt-level = 3       # Optimize for speed (~15-20% faster inference, ~1MB larger binary)
 lto = true          # Link-time optimization
 codegen-units = 1   # Single codegen unit for better optimization
 panic = "abort"     # Abort on panic (smaller binary)