DXtrade sizing: volatility only (12h default hold); fees and every-day only for DXtrade

The symbol map's fixed notional per contract is gone: every DXtrade leg is
sized from the instrument's recent volatility for the firm's target hold
(blank = 12 h), and a candidate without a volatility isn't picked. Percent
commission and every-day trading are ignored on non-DXtrade firms, which keep
measured per-contract fees and the CME calendar.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -104,6 +104,9 @@ A DXtrade firm is set up on the master's **Firms** page like any other, plus:
 
 - **Server URL** (`https://dx.tradeify247.co/dxsca-web`) and **login domain**
   (usually `default`). Changing the URL re-keys the firm's accounts.
+These settings only exist for DXtrade firms; futures firms keep their measured
+per-contract fees and the CME calendar.
+
 - **Commission %** of notional per side (Tradeify: `0.04`). The engine prices
   every fee off the fill with it.
 - **Trades every day** — weekends and CME holidays included. Such a firm runs
@@ -114,9 +117,9 @@ A DXtrade firm is set up on the master's **Firms** page like any other, plus:
   covers every root (drain and extra-day legs trade `MNQ`, so map it or use
   `*`). When a root lists several, the cheapest tradable one right now wins:
   spread plus both commissions, divided by its recent hourly volatility.
-- **Target hold hours** — size each leg's notional so its expected hold is
-  about this long (0 = the symbol map's fixed notional per $1,000 of leg), and
-  **financing %/day** if the firm charges swap.
+- **Target hold hours** — each leg's notional is sized from the instrument's
+  recent volatility so its expected hold is about this long (blank = 12 h),
+  and **financing %/day** if the firm charges swap.
 
 Per account type: **Daily loss limit** (Tradeify: 3% of size — every leg then
 carries a stop that keeps the firm's day inside it, less a 10% reserve and