Use dependency counts in CPAN failure classification

[fglock]

Summary

• Require CPAN failure classification to use cpan_port_priorities.pl when assessing the ecosystem importance of XS/native dependencies.
• Document targeted lookups, bulk shortlists, and the evidence to retain: unique runtime dependant count, recent count, and sample distributions.
• Clarify that dependency count informs prioritization but does not by itself establish root cause or porting feasibility.

Motivation

The IPC::Cache failure is caused by its native IPC::ShareLite dependency. A targeted lookup found only 3 runtime dependants, supporting deferment of an immediate native port while documenting the evidence.