feat(tracer): validated public IP seeding via seedPublicIP

[dweekly]

Summary

Implements phase 3 of Cache and Network-Transition Lifecycle from docs/CACHE-AND-TRANSITION-LIFECYCLE.md:

• PublicIPSource: Enum identifying the origin of a dynamically seeded public IP (.validatedCallerCache, .gatewayReported).
• seedPublicIP(_:source:): Actor-isolated method to provide a known-valid public IP so the next classified trace skips discovery.
  • Rejects non-global addresses (RFC 1918, CGNAT 100.64.0.0/10, link-local, loopback, ULA, multicast), malformed strings, and placeholder sentinels like "unknown".
  • Normalizes valid input to canonical inet_ntop format.
  • Associates the seed with the current cache generation so subsequent invalidations (invalidatePublicIP(), clearCaches(), networkChanged()) cleanly discard it.
  • Leaves existing discovery intact when a seed is rejected.
• Documentation: Updated SwiftFTRConfig.publicIP docstring to clarify that it is an authoritative, immutable configuration override that permanently bypasses discovery, and points callers to seedPublicIP(_:source:) for dynamic seeding across transitions.

Stacked on top of #80 (feat/network-scoped-rdns).

Testing

• Unit tests in CacheGenerationTests.swift:
  • seedValidPublicIP: Verifies seeding valid IPv4 and IPv6 strings (including non-canonical formatting normalized to canonical inet_ntop).
  • seedRejectsNonGlobalAndSentinels: Verifies rejection of RFC 1918, CGNAT, link-local, loopback, ULA, multicast, and sentinel strings like "unknown", ensuring existing cached value is untouched.
  • seedInvalidationLifecycle: Verifies that invalidatePublicIP(), clearCaches(), and networkChanged() all clear the seeded public IP.
  • seedBypassesDiscovery: Verifies that effectivePublicIPForClassification skips discovery when seeded.
• All tests pass; docc builds cleanly with --warnings-as-errors; formatting checked with swift format lint --strict.