fix(bench): enable zeph-llm testing feature for zeph-bench dev-dependencies

[bug-ops]

Summary

• AnyProvider::Mock in zeph-llm is gated behind cfg(any(test, feature = "testing")). The cfg(test) flag is not transitive across crates, so cargo test -p zeph-bench failed with 6 compilation errors in deterministic.rs.
• Fix: add zeph-llm = { workspace = true, features = ["testing"] } to [dev-dependencies] in crates/zeph-bench/Cargo.toml.
• All 91 zeph-bench unit tests now pass.

Benchmark run results (2026-04-25)

All criterion benchmarks verified working — results saved to .local/testing/bench-results-2026-04-25.md.

Crate	Bench	Status
zeph-skills	cosine_similarity, cosine_ranking	OK
zeph-memory	token_estimation	OK
zeph-core	context_building	OK
zeph-channels	markdown_performance	OK
zeph-llm	classifier (--features classifiers)	OK

Test plan

• cargo test -p zeph-bench --lib passes (91 tests)
• cargo bench -p zeph-skills runs without errors
• cargo bench -p zeph-memory runs without errors