Feedwater heat recovery: economizer, condensate, then HP-1

The order to investigate for a fired boiler (biomass such as rice
husk, or LPG): first a flue-gas economizer, second as much clean, hot
condensate back to T-1 as possible, third the heat pump HP-1 where the
press return has enough heat and the combined heating and cooling
economics justify it.

- Diagram: new ECON-1 (option, fired boilers only) between the feed
  pumps and the boiler manifold, with a bypass for an electric bank and
  the flue-gas path from the boilers; feed in 90 to 95 C, outlet and
  stack temperature are vendor figures. HP-1 is now drawn as an option
  and labelled the third measure. AT conductivity on the condensate
  return to T-1; off-spec condensate goes to the warm tank.
- Found in Newbilt's offer 240620-04: steam and cooling water share the
  mould channels, with one steam/water outlet valve and steam trap per
  mould, so the condensate is only as clean as the cooling loop. Noted
  in the T-1 detail, sustainability.md and as a Newbilt RFQ question.
- With most condensate returned (about 15 % flashes at atmospheric
  pressure, estimate) the feed tank needs only a fraction of the ~70 kW
  that heating all make-up takes, so HP-1 may not pay.
- RFQ: economizer outlet feed and stack temperature, corrosion limit,
  pressure drop; rice husk ash is high in silica and abrasive, so
  fouling, cleaning, wear protection and a bypass.
- sustainability.md: the order, before the exclusions. biomass.md: new
  "Recovering heat at the boiler".

Checked headless (scripts/pw_sc_check.py): 33 boxes, 20 tags, no
overlap or overflow, no unstyled pipes, no card or page overflow at
1600, 1258 and 390 px in all four cases, every box opens its detail, no
errors.

docs/sustainability.md

@@ -137,6 +137,21 @@ reusing it before generating anything new:
   (electric, LPG, or biomass), it supplies just the last step to 180 °C /
   10 bar from already-hot feedwater.
 
+**Order to investigate the feedwater heat recovery (October 2026).** For a
+fired boiler (biomass such as rice husk, or LPG): first a flue-gas
+economizer, recovering combustion heat into the boiler feedwater where the
+flue-gas temperature and corrosion limits allow; second, as much clean, hot
+condensate back to the feed tank as possible; third, the feedwater heat
+pump above, where the press cooling return has enough heat and the
+combined heating and cooling economics justify it. Each step shrinks the
+next one's job: with most condensate returned the feed tank needs only a
+fraction of the ~70 kW that heating all make-up would take (estimate), and
+an electric boiler has no flue gas at all. One caution on condensate: the
+press moulds take steam and cooling water through the same channels, with
+one outlet valve and steam trap per mould (Newbilt offer 240620-04), so the
+returned condensate is only as clean as the cooling loop; it is monitored
+and diverted when off-spec ([steam and cooling](steam-cooling.html)).
+
 Two honest exclusions, checked and ruled out:
 
 - **Heat pumps cannot make the final steam**: commercial steam-generating