fix: Dockerfile + CI default to x86-64-v3 (AVX2) for GitHub compatibility

- Dockerfile: ENV RUSTFLAGS="-C target-cpu=x86-64-v3" before build steps.
  Default Docker image now runs on AVX2+ hardware (GitHub CI, most servers).
  Dockerfile.avx512 still pins x86-64-v4 for production deployment.
- ci.yaml: RUSTFLAGS "-D warnings" → "-D warnings -C target-cpu=x86-64-v3"
  so CI compiles with AVX2 enabled. Previously RUSTFLAGS overrode
  .cargo/config.toml entirely, compiling at baseline x86-64 (no AVX at all).

The simd.rs polyfill detects AVX-512/AMX at runtime via LazyLock<Tier>
regardless of compile target, so the AVX2 binary still dispatches to
AVX-512 kernels on capable hardware. Compile-time v3 just means the
AVX2 fallback paths are available when runtime detection fails.

https://claude.ai/code/session_01SbYsmmbPf9YQuYbHZN52Zh

Author: claude

.github/workflows/ci.yaml

@@ -14,7 +14,7 @@ env:
   CARGO_TERM_COLOR: always
   HOST: x86_64-unknown-linux-gnu
   FEATURES: "approx,serde,rayon"
-  RUSTFLAGS: "-D warnings"
+  RUSTFLAGS: "-D warnings -C target-cpu=x86-64-v3"
   MSRV: 1.64.0
   BLAS_MSRV: 1.71.1
 

Dockerfile

@@ -31,6 +31,13 @@ COPY crates/ crates/
 COPY src/ src/
 COPY ndarray-rand/src/ ndarray-rand/src/
 
+# Default target: x86-64-v3 (AVX2) — runs on GitHub CI and most servers.
+# Use Dockerfile.avx512 for x86-64-v4 (AVX-512). ndarray's simd.rs polyfill
+# detects AVX-512 at runtime via LazyLock<Tier> even when compiled for v3;
+# compile-time v3 just means the scalar/AVX2 fallback paths are used when the
+# runtime check fails. Both paths produce identical results.
+ENV RUSTFLAGS="-C target-cpu=x86-64-v3"
+
 # Build default features
 RUN cargo build --release 2>&1 && echo "=== DEFAULT BUILD OK ==="