BOI: machinery import duty and import VAT, PVC duty relief

User, October 2026: take the BOI duty-free import and VAT exemption into
consideration.

- boi.md: import VAT goes with the BOI duty exemption (Revenue Department
  answer on the BOI OSOS FAQ, checked October 2026); what it is worth:
  ~2.75M USD of imported machinery at the start fleet (block midpoints)
  would otherwise carry ~5 % duty (Thai MFN averages 4.1 % on HS 8477,
  0 to 10 % across HS 84; estimate until the HS codes are set), ~0.14M
  USD, and 7 % import VAT, ~0.2M paid at the border and recovered later
  as input tax, mostly as a refund claim since exports are zero-rated.
- total-investment.md: the blocks are vendor prices without duty or
  import VAT, so the headline already assumes promotion.
- Fleet calculator: BOI promoted / not promoted switch and export share.
  Not promoted adds the machinery duty to capex and depreciation, shows the
  import VAT as cash at the border, and charges duty on imported PVC
  compound; promoted exempts both, with no compound duty on exported
  records and 90 % off for domestic sales (sections 36(1) and 30). A note
  under the KPIs shows what promotion saves either way. Defaults: payback
  3.1 years with BOI, 3.2 without; adverse case operating result -31k
  with BOI, -68k without. CIT and BOI CIT exemptions still not modelled.

README.md

@@ -187,7 +187,8 @@ The detail lives in dedicated documents:
   electricity and steam, labour from base salaries with social security,
   on-costs and health care for all staff, overheads, EBITDA, depreciation
   and payback, with the revenue model's adverse, central and favourable
-  cases and USD, THB or EUR; every assumption is editable, with its
+  cases, BOI promotion on or off (machinery import duty and import VAT,
+  PVC duty relief) and USD, THB or EUR; every assumption is editable, with its
   source, and a scenario can be shared by link
 - [Test site](site/README.md): `site/` builds https://cnxvinyl.lol/, the
   internal test site with demo pages from `docs/` (the plant operation