fix: gate AVX enable on XSAVE support and set CR4.OSXSAVE first

XGETBV/XSETBV require CR4 bit 18 (OSXSAVE) to be set before execution,
otherwise the CPU raises #UD (invalid opcode). Hyper-V enforces this
strictly; VirtualBox was silently tolerating it.

- Check both AVX and XSAVE CPUID flags before attempting AVX enable
- Set CR4.OSXSAVE (bit 18) before issuing XGETBV/XSETBV
- If XSAVE is not exposed by the hypervisor, AVX enable is skipped
  gracefully instead of faulting

src/cpu.pas

@@ -293,7 +293,14 @@ end;
 
 procedure enableAVX();
 begin
-    if CPUID.Capabilities1^.AVX then begin
+    if CPUID.Capabilities1^.AVX and CPUID.Capabilities1^.XSAVE then begin
+        { Enable OSXSAVE in CR4 (bit 18) - required before XGETBV/XSETBV }
+        asm
+            MOV EAX, CR4
+            OR EAX, (1 shl 18)
+            MOV CR4, EAX
+        end;
+        { Set XCR0 bits: x87 (0) + SSE (1) + AVX (2) }
         asm
             PUSH EAX
             PUSH ECX