Task 2.6: IPv6 prefix policy implementation for DISC-NG IP-diversity scoring

[srene]

Description

Implement the IPv6 prefix-aggregation policy specified by Task 2.5 (#53)
in the DISC-NG IP-diversity scoring code path. Today the IPv6 IP-tree
operates at full 128-bit granularity, which lets a dual-stack attacker
bypass the IPv4 /24 cap via many addresses within a single IPv6 /64 or
/48.

This issue covers the implementation half of the work. The
specification half — selecting the prefix length(s) and the treatment
of non-routable address ranges — is tracked as Task 2.5 (#53).

Scope

• Update netutil.IPTracker (or the DISC-NG-specific IP accounting in
  p2p/discover/topicindex/) to aggregate IPv6 addresses at the
  prefix length recommended by Task 2.5.
• Apply consistent treatment to link-local / ULA / Teredo / 6to4
  addresses per the Task 2.5 recommendation.
• Update regBucket.ips and searchBucket.ips to use the same
  policy.
• Tests covering: IPv6 same-/64 cap behaviour; IPv6 same-/48 cap
  behaviour (per recommendation); transitional-address treatment;
  IPv4-only behaviour unchanged.

Deliverables

• Code changes implementing the prefix policy.
• Unit tests covering the new behaviour.
• Documentation update if the existing comments / docs reference
  "/128" or assume per-address granularity.

Dependencies

• Blocked on Task 2.5 (Task 2.5: IPv6 prefix policy specification for DISC-NG IP-diversity scoring #53) — needs the prefix-length recommendation
  and the non-routable-address policy before implementation can start.

Phase

Phase 2 (implementation work).

Related

• Task 2.5 (Task 2.5: IPv6 prefix policy specification for DISC-NG IP-diversity scoring #53) — specification
• docs/disc-ng-compliance.md §3.2 — gap statement in the compliance
  report