src: split s_lf_riemann_solver into m_riemann_solver_lf

NFC, pure motion. State access stays host-associated via m_riemann_state; inline_riemann macros expand here (low-Mach correction), and the solver references molecular_weights directly, 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

@@ -9,6 +9,7 @@
             "m_riemann_state",
             "m_riemann_solver_hlld",
             "m_riemann_solver_hll",
+            "m_riemann_solver_lf",
             "m_muscl",
             "m_variables_conversion",
             "m_thinc"