Add restore deep trace debug mode

[sjmiller609]

Summary

• add an opt-in restore deep trace mode gated by HYPEMAN_RESTORE_DEEP_TRACE=1
• mark Resume -> guest network ack milestones in ftrace and capture /proc process/thread/io snapshots
• add a Linux-only perf test harness for Firecracker restore deep traces with guest network stage acks

Remote result

On deft-kernel-dev, 3 sampled forks showed the Resume-returned -> guest-signal window dominated by Firecracker vCPU page faults and backing-file reads, not guest netlink work:

• reconfigure_guest_network: 130ms, 342ms, 220ms
• guest_resume_network_udp_ack_wait: 110ms, 341ms, 219ms
• by guest_signal_seen: ~1.0k-1.1k minor faults, 3-29 major faults, and 19MB-140MB read_bytes on the Firecracker process, almost entirely fc_vcpu 0
• ftrace showed thousands of kvm_page_fault events plus filemap page-cache adds and block read-ahead issues during that window
• guest-internal signal -> applied remained about 4.7ms, 17.7ms, 14.2ms

Validation

• gofmt
• git diff --check
• GOCACHE=/private/tmp/hypeman-go-build go test ./lib/system/guest_agent ./lib/instances -run 'TestPatchGuestResumeNetworkMailbox|TestForkSnapshotMapsWaitForNetwork|TestRestoreDeepTracePerf' -count=1
• remote: sudo -E env ... HYPEMAN_RUN_RESTORE_DEEP_TRACE_PERF=1 HYPEMAN_RESTORE_DEEP_TRACE_PERF_ITERS=3 go test ./lib/instances -run TestRestoreDeepTracePerf -count=1 -timeout=20m -v