Energy: PEA tariff, electric vs biomass steam, PV vs solar thermal

PEA tariff schedule (May 2023 onward, still current) filed under
docs/finance/pea/; Ft 0.1623 THB/kWh for Sep-Dec 2026 from the PEA Ft page.
The plant falls under Schedule 3.2.2 (Medium General Service, TOU, 22 kV).

New section in sustainability.md: blended ~3.81 THB/kWh for two-shift
production plus 132.93 THB/kW demand charge; electric steam ~4.7M-5.5M
THB/yr at maturity against ~0.35M-0.8M of biomass fuel; self-consumed PV
worth ~3.7 THB/kWh (~4.5-7 yr payback); solar thermal not recommended;
the battery earns little from TOU arbitrage or peak shaving. biomass.md,
scale.md, the utilities CSV note and the README follow.

docs/biomass.md

@@ -80,8 +80,18 @@ storage are additional.
 - The 60 rai tilts the economics and the story further toward biomass, but
   only after the land-title check clears
 - The electric boiler remains the alternative: cleanest permitting, no fuel
-  logistics, but effectively grid-powered steam at a ~300 kW draw
+  logistics, but effectively grid-powered steam at a ~300 kW draw. On
+  PEA's time-of-use tariff that costs ~4.7M to 5.5M THB a year at mature
+  load against ~0.35M to 0.8M of fuel for biomass, and ~1.4M to 1.7M
+  against ~0.1M to 0.2M at early load, because the demand charge follows
+  the boiler's full-power draw (estimates; see the
+  [grid tariff](sustainability.md#grid-tariff-pea-and-what-it-decides)).
+  The electric boiler is the more efficient machine; biomass is the far
+  cheaper heat
 - A staged path is possible: start electric (fast, clean permitting), plant
   the coppice now (first harvest in 2 to 4 years), and add the biomass
   boiler at the automated-line expansion when demand and the fuel supply
-  have both matured
+  have both matured. At the tariff above that is a permitting and logistics
+  choice, not a cost one: every year on electric steam costs roughly 1.2M
+  to 5M THB more than biomass, so the switch should come as soon as the
+  permit and a fuel contract are in place