refactor: config plumbing and coding-style cleanup (no functional changes)

[alfxu-amd]

Summary

A refactor-only pass. No functional changes — behavior, numerics, and public
APIs are unchanged; this is about structure and coding style.

Config handling

LM_Config had accumulated model-specific members that were all just copies of
values already present in _json_config. Every new model meant another field on
a struct passed by value across the DLL boundary, and the two mirrored
lm_config.hpp copies had to be kept layout-identical by hand.

LM_Config now carries only paths, name, version, and the raw _json_config,
with get<T>(key, default) and sub(key) accessors. Model code reads what it
needs directly. Note that sub()/get<T>() exist rather than plain
nlohmann .value() because real configs carry JSON nulls (e.g.
"sliding_window": null), which .value() throws on.