Fix Windows x86 calling convention mismatch for HPDF error handler

[Copilot]

On Windows x86 with -DHPDF_DLL, HPDF_Error_Handler is typedef'd with HPDF_STDCALL (__stdcall), but error_handler was declared with the default __cdecl, causing a hard compile error when passing it to HPDF_New. This is a no-op on x64 where the two conventions are unified.

Change

• Added HPDF_STDCALL to the error_handler function definition in docraft_haru_backend.cc so its type exactly matches HPDF_Error_Handler:

// Before
void error_handler(HPDF_STATUS error_no, HPDF_STATUS, void *) { ... }

// After
void HPDF_STDCALL error_handler(HPDF_STATUS error_no, HPDF_STATUS, void *) { ... }

HPDF_STDCALL is already defined by hpdf_types.h — expands to __stdcall on x86 Windows with HPDF_DLL, empty string everywhere else.

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