Add comprehensive lib package tests to drive coverage toward 100%

[Codex]

The lib package lacked unit tests, preventing confident changes and high coverage.

• Test scaffolding: Added shared helpers and mock converters to validate behaviors without external deps (lib/helpers_test.go).
• Core helpers: Covered URL fetchers, ignore options, and wanted-list JSON parsing (common_test.go, lib_test.go).
• Config parsing: Exercised config creator registration and converter unmarshalling for input/output paths, including hujson handling (config_test.go, converter_test.go, instance_test.go).
• Container semantics: Verified add/merge/remove branches, lookup filters, builder creation, and error paths across IPv4/IPv6 permutations (container_test.go).
• Entry semantics: Tested prefix processing for multiple input types, builder error propagation, marshal methods with ignore options, and edge/invalid cases (entry_test.go).

Example:

c := NewContainer()
e := NewEntry("ZONEA")
_ = e.AddPrefix("10.3.0.0/16")
_ = c.Add(e)
names, ok, _ := c.Lookup("10.3.5.1")
fmt.Println(names, ok) // [ZONEA] true