[#16]:svarga:refactor, std::tuple type-axis (throughput<Tuple>) and retire bench::types<>

[steven-varga]

Closes #16.

The type axis is now a plain std::tuple passed as an explicit template argument; the bench::types<Ts...> marker is retired (redundant — static_for already iterates a std::tuple).

Engine

• New template<class Tuple, class... args_t> requires impl::is_tuple<Tuple> void throughput(args_t...) — called bench::throughput<std::tuple<A,B,C>>(...). Same fold/timing/stats/rows as before (one row per (type,x), "<name>/<typelabel>").
• Disambiguation: Tuple is the leading explicit template arg, constrained by a new impl::is_tuple concept (is_specialization_of + remove_cvref). It's never deduced, so a plain throughput(name{...}, ...) falls through to the scalar overload; throughput<std::tuple<...>>(...) selects this one. Both verified.
• Deleted struct types and the old marker overload.

Migrated call sites (grep -rn 'bench::types' → empty)

test/type_dispatch.cpp, examples/type_dispatch.cpp, examples/heatmap.cpp, README.md, and examples/memcpy_bandwidth.cpp (rewritten to the designed typed-tuple form: std::tuple<uint8_t,uint16_t,uint32_t,uint64_t> with typed static buffers and a [&]<class T> body).

Verification

Build green (-DBENCH_BUILD_EXAMPLES=ON); ctest 5/5; memcpy_bandwidth/type_dispatch/heatmap examples all run.

Base: staging.