Default object-key directory for per-symbol price files. Override at the entry
point (e.g. YUZU_PRICES_DIR) for a custom layout — the engine stays generic.
Read the per-symbol price file for each symbol (probing .csv.gz, then
.parquet when that feature is on, then .csv; format detected from
content), extract field, filter to [from, to] (inclusive, YYYYMMDD), and
assemble a Panel: rows = sorted union of all kept days, columns = symbols in
the given order. Missing cells (and symbols with no file) are NaN. dir
defaults to PRICES_DIR at call sites.