fix(win): compile cleanly with MinGW GCC

[Oxygen56]

Fixes #1225

What does this PR do?

Makes the Windows sources compile cleanly with MinGW GCC when warnings are treated as errors.

• Removes a redundant DWORD range comparison that GCC reports as always false on 64-bit Windows. The existing MAXDWORD guard keeps the subsequent +1 safe, while calloc retains allocation-overflow handling.
• Converts dynamically resolved Windows functions through GCC's generic void (*)(void) function-pointer type before assigning their concrete signatures.
• Applies the same conversion to the Windows IPC regression test helpers.

Root cause

MinGW defines FARPROC as an unspecified-parameter function pointer. Direct casts from it to concrete Win32 function signatures trigger -Wcast-function-type. Separately, a DWORD can never exceed the 64-bit SIZE_MAX-based bound, so -Wtype-limits rejects that comparison under -Werror.

Verification

• Unmodified origin/main reproduced both warning classes with x86_64-w64-mingw32-gcc 16.1.0 -Wall -Wextra -Werror.
• The affected production translation units compile cleanly with the same MinGW GCC flags after the fix.
• The Windows IPC test translation unit compiles cleanly for the affected warning class; -Wno-stringop-overflow was used only to isolate pre-existing unrelated diagnostics in that file.
• make -f Makefile.cbm test — 6776 passed, 4 skipped.
• make -f Makefile.cbm lint-ci — passed.
• scripts/check-dco.sh origin/main..HEAD — passed.

Checklist

• Every commit is signed off (git commit -s) — required, CI rejects
  unsigned commits (DCO, see CONTRIBUTING.md)
• Tests pass locally (make -f Makefile.cbm test)
• Lint passes (make -f Makefile.cbm lint-ci)
• New behavior is covered by a test (reproduce-first for bug fixes)