NLL to Polonius migration

Find the best ownership design, then establish whether it needs Polonius. Retiring a workaround, improving an API, and changing the supported toolchain are separate decisions. A valid audit can recommend useful refactors and no compiler migration.

Start from the router's shared Polonius Alpha project posture reference: it defines how to detect the project's borrow-checker posture, the compile canary for ambiguous toolchains, and the boundary the ordinary Rust skills consume. This skill builds on that posture rather than restating it.

Status and framing (read before touching code)