src: split s_hll_riemann_solver into m_riemann_solver_hll

NFC, pure motion. State access stays host-associated via m_riemann_state; the inline_riemann include moves with the solver since its macros expand here (roe_avg pulls in get_species_enthalpies_rt, gas_constant, molecular_weights, hence the m_thermochem only-list). Core re-exports the entry point through its existing public list. Statement-multiset union vs the pre-split file differs only by module boilerplate; GPU-directive census md5 unchanged; declare-target scoping verified for all files.

Author: sbryngelson

docs/module_categories.json

@@ -8,6 +8,7 @@
             "m_riemann_solvers",
             "m_riemann_state",
             "m_riemann_solver_hlld",
+            "m_riemann_solver_hll",
             "m_muscl",
             "m_variables_conversion",
             "m_thinc"