Loopback node-segment template and startup migration check

[ben-malbeclabs]

Add flex-algo node-segment lines to loopback template, one per entry in flex_algo_node_segments. Conditional on FlexAlgoNodeSegments being non-empty — if empty, only the algo-0 node-segment line is rendered.

Controller MUST check at startup with enabled: true that no Vpnv4 loopback has an empty flex_algo_node_segments list. If any are found, enabled: true MUST be treated as a no-op for that startup cycle — no flex-algo config pushed to any device. Controller MUST emit a prominent error identifying unset loopbacks by pubkey and direct the operator to run the migration command.

Milestone: RFC-18: Controller — Topology Config
RFC: #3288

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Files affected:

• controlplane/controller/internal/controller/ (loopback template, startup check)

Size: Small

Tests:

• Loopback with flex_algo_node_segments entries renders one node-segment ... flex-algo <name> line per entry
• Loopback with empty flex_algo_node_segments produces only the algo-0 node-segment line
• Startup check fires and emits correct error when any Vpnv4 loopback has empty list
• Startup check passes and flex-algo config is applied when all loopbacks are migrated

Blockers: #3351 (IS-IS TE templates), #3349 (migration command)