Drop-in wrapper around Pgvector.Ecto.Vector with a real equal?/2.

Pgvector.Ecto.Vector inherits Ecto's default equal?/2 (plain ==), so change detection compares terms structurally. Whenever the value loaded from the database isn't the exact representation being written back — a custom Postgrex types module decoding to a different struct, or list-vs-%Pgvector{} round trips — a byte-identical embedding always dirties the changeset, turning idempotent re-stores into write amplification plus HNSW index churn.

This type delegates type/cast/load/dump to Pgvector.Ecto.Vector (identical wire behavior) and compares values by their encoded binary, so byte-identical vectors are equal regardless of representation.

Values it can't encode (an opaque struct from a custom decoder that is neither enumerable nor castable) fall back to structural comparison, which is today's behavior rather than a crash.

If pgvector-elixir ships equal?/2 upstream (pgvector-python's Vector.__eq__ is the family precedent), this module becomes a no-op layer that can be dropped in a major release.