[adr-031] EmbedderRegistry implementation (integration codex MAJ-3)

[ohdearquant]

Integration codex ADR contract review MAJ-3.

ADR-031 calls for EmbedderRegistry — pack-extensible embedding model registry. Currently only the embedding model registry table exists (V14 from c20); the runtime-level EmbedderRegistry trait + registration pattern is not implemented.

Follow-up to PR #349 launch — defer to phase-2 work alongside ADR-043 D2-D6 (#368, #380).