fix(docker): ship declared example/bench sources in build context

The Railway/Docker build aborted at manifest parse:

  error: failed to parse manifest at `/app/Cargo.toml`
  Caused by:
    can't find `amx_gemm_bench` example at `examples/amx_gemm_bench.rs` ...

Root cause: `.dockerignore` excluded `examples/` and `benches/` from the
build context, but Cargo.toml declares explicit [[example]]/[[bench]]
targets. Cargo validates that every explicitly-declared target's source
file exists while *parsing* the manifest — even for a lib-only build — so
`cargo build` aborts before compiling anything when those files are absent.
`amx_gemm_bench` was just the first missing target reported; all 18
examples and 13 root benches were stripped, and the ndarray-rand workspace
member's [[bench]] "bench" was also missing (its benches/ dir was never
copied — only Cargo.toml + src/).

Fix:
- .dockerignore: stop excluding examples/ and benches/ so they are present
  in the build context (kept an NB comment explaining why).
- Dockerfile + Dockerfile.avx512: COPY examples/, benches/, and
  ndarray-rand/benches/ before the build. These are NOT compiled by the
  default `cargo build` (examples/benches require --examples/--benches), so
  this adds only source bytes, not build time.

Verified by staging the exact Docker COPY set locally: without the dirs,
`cargo build` reproduces the parse error; with them, parse succeeds and the
build advances past target validation into dependency/lib compilation.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Kq2xzjPj9p2eoP64uy3Btg

Author: claude

.dockerignore

@@ -4,6 +4,10 @@ target/
 *.md
 !README*.md
 .github/
-benches/
-examples/
 scripts/
+# NB: benches/ and examples/ are intentionally NOT ignored. Cargo.toml declares
+# explicit [[bench]] and [[example]] targets, and cargo validates that every
+# declared target's source file exists when it parses the manifest — even for a
+# lib-only build. Excluding these dirs makes `cargo build` fail at manifest
+# parse with "can't find <name> example/bench". They are not compiled by the
+# default build, so the cost of shipping them in the context is just their size.

Dockerfile

@@ -34,6 +34,16 @@ COPY crates/ crates/
 COPY src/ src/
 COPY ndarray-rand/src/ ndarray-rand/src/
 
+# Cargo.toml (root) and ndarray-rand/Cargo.toml (a workspace member) declare
+# explicit [[example]]/[[bench]] targets. Cargo validates that every declared
+# target's source file exists while parsing the manifest — even for a lib-only
+# build — so these dirs must be in the context or `cargo build` fails at parse
+# with "can't find <name> example/bench". They are NOT compiled here (the default
+# build skips examples/benches), so this only adds source bytes, not build time.
+COPY examples/ examples/
+COPY benches/ benches/
+COPY ndarray-rand/benches/ ndarray-rand/benches/
+
 # 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;

Dockerfile.avx512

@@ -32,6 +32,16 @@ COPY crates/ crates/
 COPY src/ src/
 COPY ndarray-rand/src/ ndarray-rand/src/
 
+# Cargo.toml (root) and ndarray-rand/Cargo.toml (a workspace member) declare
+# explicit [[example]]/[[bench]] targets. Cargo validates that every declared
+# target's source file exists while parsing the manifest — even for a lib-only
+# build — so these dirs must be in the context or `cargo build` fails at parse
+# with "can't find <name> example/bench". They are NOT compiled here (the default
+# build skips examples/benches), so this only adds source bytes, not build time.
+COPY examples/ examples/
+COPY benches/ benches/
+COPY ndarray-rand/benches/ ndarray-rand/benches/
+
 # AVX-512 pinned: compile-time dispatch, everything inlined
 ENV RUSTFLAGS="-C target-cpu=x86-64-v4"