Transactional registration for external codec packages.

A package application can publish its complete codec set and table-provider set with one ownership token:

{:ok, token} =
  Iconvex.Extension.register(
    [
      MyPackage.Codec,
      {MyPackage.OtherCodec,
       canonical: "MY-SOURCE-OTHER", aliases: ["MY-OTHER"]}
    ],
    owner: :my_package,
    priority: 0,
    table_providers: [:my_table]
  )

# Store `token` in the application state.
:ok = Iconvex.Extension.unregister(token)

An atom in :table_providers uses the package :owner as its OTP application. {table_id, application} may be used when the table files live in a different application.

Providers become available before the codec routes are committed. Registration failure rolls back every provider acquired by this transaction. Shutdown removes codec routes before providers, so a concurrent conversion cannot resolve a new codec against a missing provider. The route snapshot makes each commit or removal externally atomic.

The token is opaque and exact. Passing an unknown or stale token is an idempotent no-op, and provider cleanup uses the underlying ownership references so it cannot remove a later replacement. Repeating an identical registration for the same owner adopts the committed token, including after a supervised registry-worker restart. A different active transaction for that owner is rejected.

Managed name claims are deterministic: canonical claims outrank aliases, then the larger integer priority wins within the same claim kind. Equal-rank claims by different modules are rejected as ambiguous. Priorities should be fixed package metadata, not runtime configuration.