doublezero-admin migration command

[ben-malbeclabs]

One-time migration command covering two tasks:

1. Links — iterate all existing Link accounts with link_topologies = [] and set link_topologies[0] to the UNICAST-DEFAULT TopologyInfo pubkey.
2. Vpnv4 loopbacks — iterate all existing Interface accounts with loopback_type = Vpnv4 and allocate a FlexAlgoNodeSegment entry (topology pubkey + allocated node segment index from SegmentRoutingIds ResourceExtension) for each known TopologyInfo account. Existing node_segment_idx assignments (algo-0) are unchanged.

The command MUST be idempotent — re-running skips already-migrated accounts. Must emit a summary on completion (e.g. migrated 12 links, 4 loopbacks; skipped 3 already migrated). A --dry-run flag MUST be supported.

Milestone: RFC-18: CLI
RFC: #3288

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Files affected:

• controlplane/doublezero-admin/src/cli/ (new migration subcommand)

Size: Medium

Tests:

• Migration skips already-migrated links and loopbacks
• Allocates correctly for unset links and loopbacks
• Dry-run output matches expected changes without applying
• Idempotent on re-run
• Summary output correct

Blockers: #3344 (TopologyInfo account)