fix two-phase lookup and test assertion in pool_type_impl<T&> fix (#504 follow-up)

[PavelGuzenfeld]

Follow-up to #674 (merged as e4fdeb2), which introduced two issues that break CI on all platforms.

Problem 1: two-phase name lookup

pool_type_impl<T&>(const init&, const TObject&) calls try_get<T> in a member-initialiser list. try_get is declared later in the same namespace; without a prior declaration the compiler cannot recognise try_get<T>(...) as a template call at definition time (C++ [temp.dep.res] §13.8.3). All four CI platforms now fail with:

include/boost/sml.hpp:658: error: 'try_get' was not declared in this scope

Fix: add forward declarations for pool_type, missing_ctor_parameter, and all try_get overloads immediately before the #if defined(BOOST_SML_CREATE_DEFAULT_CONSTRUCTIBLE_DEPS) block.

Problem 2: invalid is<>() assertion in the regression test

The test called:

expect(sm.is<sml::state<sub504>>(sml::X));

sml::state<sub504> is a value expression, not a type; sm::is<T> requires a raw class type (T::type must be valid). Clang, GCC, and MSVC all reject this with "invalid template argument".

Fix: remove the assertion. The action lambda [](dep504& d) { expect(99 == d.val); } is the actual liveness check — if the reference were dangling, reading d.val would be UB/crash, which is what the test is designed to catch.

Tests

make test CXX=g++ CXXSTD=c++17 and CMake Debug build both pass with zero errors.

[PavelGuzenfeld]

Please merge this one first and then I'll debase all the rest to fix the CI.

[PavelGuzenfeld]

Note: the upstream master branch CI is also currently failing with the same dependencies.cpp:396 error (run 26344508978) — this PR fixes both the PR regression and the broken master.