Custom data source realtime quotes were missing change_pct because _build_index_quotes only kept pre-existing columns. TickFlow path computes it in _fetch_full_market_quotes, but custom source path bypasses that computation.
- _build_index_quotes computes change_pct/change_amount from last_price/prev_close when not provided by the source
- guards prev_close=0 (Polars produces inf, invalid JSON)
- aligns with TickFlow path and _fallback_index_quotes_from_daily