link_topologies: Vec<Pubkey> on Link account

[ben-malbeclabs]

Append link_topologies: Vec<Pubkey> (cap 8) to the serviceability Link account. Foundation-only write restriction. Validate pubkeys against TopologyInfo accounts on set.

Milestone: RFC-18: Onchain Foundation
RFC: #3288

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Files affected:

• smartcontract/programs/doublezero-serviceability/src/state/link.rs
• smartcontract/programs/doublezero-serviceability/src/processors/link/

Size: Small

Tests:

• Invalid pubkey rejected
• Empty vec accepted
• Cap of 8 enforced
• Non-foundation key rejected

Blockers: #3344 (TopologyInfo account)