test: prove VirtualBox methods observe the same VM state

[Wenzel]

Closes #286.

Adds opt-in cross_method_consistency, requiring VBOX_TEST_CROSS_METHOD=1, exact filter, VBoxVMI method and 2 vCPUs. It attaches MachineDebugger and VBoxVMI concurrently to the same paused VBoxHeadless process (test-only combined branch Wenzel/virtualbox:vmi/cross-method-test) and:

• compares exact complete registers for each vCPU;
• compares every byte in live PGM RAM ranges directly, no core intermediary;
• proves memory writes MD→VBoxVMI and VBoxVMI→MD, then restores and verifies through both;
• proves SYSENTER register writes in both directions, restores and verifies through both;
• uses unwind guards for byte/register restoration and VM resume.

Ordinary virtualbox_ suites exclude it; unfiltered runs without opt-in print SKIPPED; filtered invocation without opt-in fails. integration_filter covers selection behavior.

Verified live:

RAM	Bytes compared	Above 4 GiB	vCPUs	Result
512 MiB	536,477,696	0	2	pass
5120 MiB	5,368,315,904	1,610,612,736	2	pass

The 5 GiB run writes/restores page 0x15fff6000 above 4 GiB. fmt, clippy -D warnings and integration_filter pass.