fix: add sml::deps<Ts...> policy for explicit pool deps (issue #437)

[PavelGuzenfeld]

Problem

When all actions and guards use generic 4-arg lambdas to access deps manually:

auto action = [](auto /*e*/, auto& /*sm*/, auto& deps, auto& /*states*/) {
    sml::aux::get<MyDep&>(deps).value = 99;
};

…MyDep never appears in any explicitly-typed parameter list, so dep_list (computed via get_deps_t → args_t) is empty. The pool deps_t never includes MyDep, and the static_cast from the user-provided pool to pool_type<MyDep&> fails to compile:

error: static_cast from 'aux::pool<>' to 'aux::pool_type<MyDep&>' is not valid

Closes #437.

Fix

Introduce sml::deps<Ts...> as a new SM policy that widens the deps pool to include explicitly named types:

MyDep dep;
sml::sm<MyTable, sml::deps<MyDep>> sm{dep};

Implementation

1. back::policies::explicit_deps<Ts...> — new policy struct, follows same aux::pair<Tag__, Self> pattern as logger, thread_safe, etc.
2. sm_policy::explicit_deps_policy — picked up via the existing get_policy mechanism.
3. get_explicit_deps<P> helper — converts explicit_deps<Ts...> → type_list<Ts&...> (with same const T& → T& normalization as ignore::non_events); no_policy → empty list.
4. sm::deps_t — joins explicit_dep_list before the auto-deduced dep_list; unique_t deduplicates when both list the same dep.
5. Public alias — template<class... Ts> using deps = back::policies::explicit_deps<Ts...>;

The design is purely additive: existing SMs without the policy are unchanged.

Verified

• GCC 14 C++17: 29/29 ft ✅
• GCC 14 C++20: 29/29 ft ✅
• MSVC 19.51 C++20: 34/34 ✅

Companion test PR: #691