fix(container): copy benches/ so root manifest parses in minimal image

cargo validates every declared target when parsing Cargo.toml, even for
`--bin echidna`. Root manifest declares explicit [[bench]] targets
(proof_benchmarks, routing_benchmarks at benches/*.rs); without benches/
in the build context the parse fails with
`can't find 'proof_benchmarks' bench`. Verified locally via
`cargo metadata --no-deps` against the exact Containerfile copy set.
Completes the minimal-image build-context fix (cf. prior crates/ commit).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

Containerfile

@@ -32,6 +32,13 @@ COPY src/interfaces ./src/interfaces
 # must be in the build context too — omitting it fails the build with
 # `failed to read /build/crates/echidna-core/Cargo.toml`.
 COPY crates ./crates
+# Root Cargo.toml declares explicit [[bench]] targets (proof_benchmarks,
+# routing_benchmarks at benches/*.rs). Cargo validates every declared
+# target when it parses the manifest — even for `--bin echidna` — so the
+# benches/ sources must exist in the build context or parse fails with
+# `can't find 'proof_benchmarks' bench`. (tests/ and examples/ are
+# auto-discovered and tolerate absence, so they are intentionally not copied.)
+COPY benches ./benches
 
 RUN cargo build --release --bin echidna