activator,sdk: add topology event handler and update Python/TypeScript SDKs for RFC-18

[ben-malbeclabs]

RFC-18 flex-algo · PR 4 of 5 · see rfcs/rfc-0018-flex-algo.md
Depends on: #3512 (PR 2)
Series: #3497 · #3512 · #3513 · #3514 · #3515

Summary of Changes

• Adds process_topology_event to the activator: when a TopologyInfo account is created or updated, the activator calls BackfillTopology on all devices that have an activated VPNv4 loopback (idempotent — skips devices that already have a segment for this topology)
• Adds --enable-flex-algo flag to the activator (default: off); topology events are ignored unless the flag is set
• Updates Python SDK to deserialize TopologyConstraint, TopologyInfo, and FlexAlgoNodeSegment, and reads flex_algo_node_segments from V2 interface accounts
• Updates TypeScript SDK to deserialize FlexAlgoNodeSegment from V2 interface accounts and adds a configurable request timeout to the RPC client
• Updates binary fixtures for device, link, and tenant accounts to include the new V2 interface format fields

Diff Breakdown

Category	Files	Lines (+/-)	Net
Core logic	7	+315 / -27	+288
Scaffolding	5	+17 / -5	+12
Tests	1	+3 / -1	+2
Fixtures	3	Bin changes

Mostly core logic concentrated in the new topology event handler and Python SDK deserialization.

Key files (click to expand)

• activator/src/process/topology.rs — new: topology event handler; lists all active devices, filters to those with activated VPNv4 loopbacks, batches BackfillTopology calls
• activator/src/process/iface_mgr.rs — calls backfill after a VPNv4 loopback is activated, so newly provisioned devices immediately get node segments for existing topologies
• activator/src/process/device.rs — topology-aware device processing
• activator/src/processor.rs — wires in process_topology_event for AccountData::Topology events; adds enable_flex_algo guard
• sdk/serviceability/python/serviceability/state.py — adds TopologyConstraint enum, TopologyInfo dataclass, FlexAlgoNodeSegment dataclass; reads flex_algo_node_segments vec from V2 interface accounts
• sdk/serviceability/typescript/serviceability/state.ts — adds FlexAlgoNodeSegment type; reads it from V2 interface accounts
• sdk/serviceability/typescript/serviceability/rpc.ts — adds configurable request timeout

Testing Verification

• cargo test -p doublezero-activator — 76 tests pass
• Python SDK: 115 tests pass (pytest, including fixture deserialization)
• TypeScript SDK: 138 tests pass (bun test, including fixture deserialization)
• cargo clippy -- -D warnings clean after fixing for_kv_map lint in topology.rs