Optimize restore network setup

[sjmiller609]

Summary

• collapse CreateAllocation from separate name/IP/MAC allocation scans to one ListAllocations pass
• replace bridge isolation subprocess with netlink.LinkSetIsolated
• avoid the post-create TAP LinkByName lookup by reusing the Tuntap object populated by LinkAdd
• adjust perf output for the optimized path: add network_list_allocations_ms and remove the removed link_lookup_tap field

Benchmarks

From the prior 20-sample deft run on the VMGenID/mailbox fast-return path:

• fork_total_ms: avg 175.9, median 156, p90 238, p95 257, max 268
• restore_network_ms: avg 5.4, median 3, p90 7, p95 18, max 19
• network_create_tap_ms: avg 4.0, median 2.5, p90 5, p95 16, max 18
• network_link_add_ms: avg 2.3, median 1, p90 3, p95 14, max 16
• network_set_isolation_ms: 0ms in final sample

Validation

• git diff --check
• GOCACHE=/private/tmp/hypeman-go-build GOOS=linux go test -c -o /private/tmp/hypeman-network.test ./lib/network
• GOCACHE=/private/tmp/hypeman-go-build go test ./lib/instances -run 'TestVMGenIDResumeSignal|TestApplyForkTargetStateStoppedRefreshesSnapshotForkCID' -count=1