Replace fixed-size core-variable parallel arrays with MemAlloc'd LibretroCoreVariable array

Resolves the existing // TODO: Switch these to MemAlloc'ed strings. in LibretroCoreData. Follow-up from #180.

Core options storage is currently six parallel fixed-size arrays:

char variableKeys[LIBRETRO_MAX_CORE_VARIABLES][LIBRETRO_CORE_VARIABLE_KEY_LEN];
char variableValues[LIBRETRO_MAX_CORE_VARIABLES][LIBRETRO_CORE_VARIABLE_VALUE_LEN];
char variableLabels[LIBRETRO_MAX_CORE_VARIABLES][LIBRETRO_CORE_VARIABLE_LABEL_LEN];
char variableValuesList[LIBRETRO_MAX_CORE_VARIABLES][LIBRETRO_CORE_VARIABLE_VALUES_LEN];
char variableDisplayList[LIBRETRO_MAX_CORE_VARIABLES][LIBRETRO_CORE_VARIABLE_VALUES_LEN];
char variableTooltips[LIBRETRO_MAX_CORE_VARIABLES][LIBRETRO_CORE_VARIABLE_TOOLTIP_LEN];
bool variableVisible[LIBRETRO_MAX_CORE_VARIABLES];
unsigned variableCount;

Issues:

• Bloats sizeof(LibretroCoreData) by hundreds of KB regardless of actual variable count.
• Makes the memset(0) reset enabled by #180 unnecessarily expensive.
• Six parallel arrays are harder to reason about than a single struct.

Proposed:

typedef struct LibretroCoreVariable {
    char key[LIBRETRO_CORE_VARIABLE_KEY_LEN];
    char value[LIBRETRO_CORE_VARIABLE_VALUE_LEN];
    char label[LIBRETRO_CORE_VARIABLE_LABEL_LEN];
    char valuesList[LIBRETRO_CORE_VARIABLE_VALUES_LEN];
    char displayList[LIBRETRO_CORE_VARIABLE_VALUES_LEN];
    char tooltip[LIBRETRO_CORE_VARIABLE_TOOLTIP_LEN];
    bool visible;
} LibretroCoreVariable;

typedef struct LibretroCoreData {
    // ...
    LibretroCoreVariable* variables;
    unsigned variableCount;
    unsigned variableCapacity;
    // ...
} LibretroCoreData;

MemAlloc on first variable, grow as needed, MemFree in CloseLibretro before the trailing ResetLibretroCoreState() memset (so the memset just nulls the pointer).

Touches: LibretroInitCoreVariable, SetLibretroCoreOption, GetLibretroCoreOption, env handlers for RETRO_ENVIRONMENT_SET_VARIABLES / SET_CORE_OPTIONS / SET_CORE_OPTIONS_V2 / option visibility, menu rendering in raylib-libretro-menu.h.

Start by locating LibretroCoreData and the LibretroInitCoreVariable, SetLibretroCoreOption, and GetLibretroCoreOption paths, then trace the RETRO_ENVIRONMENT_SET_VARIABLES, SET_CORE_OPTIONS, SET_CORE_OPTIONS_V2, and option-visibility handlers. Review menu rendering in raylib-libretro-menu.h and the CloseLibretro/reset sequence. Done means variable storage is dynamically allocated and freed, all listed callers use the struct array, and reset leaves the state valid.