Compute embedding vectors for records using an Embedding_Strategy.
This is a backend-specific generic that handles data retrieval,
text assembly, and embedding computation via tidyllm.
Embedding is the expensive part of a vector match, so each record is embedded
once and the vector is reused on later calls. The data.table and tibble
backends keep a per-session cache keyed by model and record content; the
DuckDB backend reuses through its persisted embeddings column. Reuse is
controlled by the joinery.embedding_reuse and joinery.embedding_cache_dir
options (see joinery package options) and can be cleared with
clear_embedding_cache().
Value
A backend-specific table with columns: id and embedding
(where embedding contains numeric vectors).
