refactor(traits-2): split containers.rs by backend

The file carried two independent backends behind one banner comment: the
Vec coordinate list (K: Eq + Clone, linear scan) and the HashMap hash join
(K: Indexable). Each now owns a file with its own imports, so a change to
one backend's cost model is not read past the other. The tests stay in one
module — they share the Key/batch fixtures and one test spans both
backends by design.

Prose is preserved verbatim: the orphan-rule friction note, the
smaller-side `overlap` rationale, the Lean cross-references, and the
`key_hash()` warm-up comment.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: Roger-luo