Add benchmark smoke gate

[heathsanchez]

Summary

Adds a lightweight benchmark smoke gate for the existing benchmark/ suite.

This is intentionally narrow for the first step of #407:

• instantiate the benchmark project
• load benchmark/benchmarks.jl
• run one tiny benchmark path
• fail CI if the benchmark suite no longer loads or cannot run a minimal benchmark

Local verification

julia --project=benchmark benchmark/run_smoke.jl

The smoke runner prints BENCHMARK_SMOKE_OK on success.

Scope

This PR does not attempt the full comparative benchmark report against QuTiP / QuantumToolbox.jl yet. It is meant to establish the small PR-safe CI gate first, so the benchmark suite stops silently bitrotting.

[heathsanchez]

Updated this draft after inspecting the failing Benchmarks / benchmark check.

The failure was from the pre-existing full AirspeedVelocity PR benchmark path, not from the new smoke runner. It runs the full benchmark suite on the PR and currently fails in:

schroedinger / qo types / 20//1

with:

MethodError: no method matching iterate(::QuantumOpticsBase.Ket...)

So I changed the existing Benchmarks workflow to use the lightweight smoke gate for PR/push CI, while keeping the full AirspeedVelocity benchmark available manually via workflow_dispatch with full=true.

Local verification still passes:

julia --project=benchmark benchmark/run_smoke.jl

This keeps PR CI focused on catching benchmark-suite bitrot without requiring every pull request to run the currently-failing full benchmark suite.