unicast_drained: bool on Link account + CLI

[ben-malbeclabs]

Append unicast_drained: bool to the serviceability Link account, defaulting to false. Contributor-writable — a contributor may drain their own link from all constrained unicast topologies as a capacity management tool. Orthogonal to link.status.

doublezero link update --code <CODE>     --unicast-drained true|false
doublezero link update --pubkey <PUBKEY> --unicast-drained true|false

When unicast_drained = true, the controller appends UNICAST-DRAINED to the interface's traffic-engineering administrative-group alongside permanent topology tags. Permanent tags are never modified. doublezero link get and doublezero link list MUST display unicast-drained: true/false.

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/
• smartcontract/cli/src/link/update.rs

Size: Small

Tests:

• Defaults to false on existing accounts deserialized from pre-upgrade binary data
• Contributor can set unicast_drained = true on their own link
• Contributor cannot set unicast_drained on a link they do not own
• Setting unicast_drained = true does not modify link_topologies or link.status
• CLI resolves correctly by both --code and --pubkey
• link get and link list display the field

Blockers: None

[ben-malbeclabs]

Implemented in #3497 (RFC-18 PR 1).