fix(eims): match MoR's payload rules found by live rejections

Three live attempts turned six guesses into facts. Each fix below is the
gateway's own words, not a reading of the collection.

DocumentNumber and InvoiceCounter move differently, because MoR constrains
them differently. The counter must not skip -- "Invoice counter is not
correct. expected : 1" -- so a definitively refused document hands it back.
The document number must not repeat, so the attempt burns it. Both stay spent
after an ambiguous result, where MoR may have stored the document.

NatureOfSupplies is normalised to MoR's exact lowercase constant and rejected
outright if it is neither 'goods' nor 'service'; its schema branches on this
as a oneOf, so "Service" invalidated the whole ItemList.

Buyer region resolves through a name->code map and now FAILS locally when
unmapped. MoR validates Region against ^[0-9]{1,3}$ on both the seller and
buyer sides, so a name can never be sent and a guessed code on a tax document
is worse than refusing to file.

Seller phone, email, region and wereda are checked against MoR's own regexes
before anything is sent, so a placeholder like "_" fails locally instead of
costing a request and a counter.

EIMS_TAX_CODE stays required and unset in .env.example: the choice between
VAT0 (zero-rated) and VATEX (exempt) is a tax position awaiting finance, and
MoR's enum is recorded there for whoever decides.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

apps/edr-freight-api/.env.example

@@ -172,8 +172,10 @@ EIMS_SELLER_LOCALITY=
 # Tax treatment — REQUIRES FINANCE SIGN-OFF. The application models no tax at all
 # (invoice.taxAmount is always 0), so nothing here is defaulted: registration fails
 # locally, naming the missing variables, until these are set.
-# MoR enum: TOT10 TOT2 VAT15 VWHT TWHT VATEX VATWH WHOP2 WTHOI VAT0 VWTH
-EIMS_TAX_CODE=VAT0
+# Required, and deliberately unset: the choice is a tax position, not a default.
+# MoR's enum (from its own 400): TOT10 TOT2 VAT15 VWHT TWHT VATEX VATWH WHOP2 WTHOI VAT0 VWTH
+# Pending finance confirmation of VAT0 (zero-rated) vs VATEX (exempt).
+EIMS_TAX_CODE=
 EIMS_TAX_RATE_PERCENT=0
 EIMS_EXCISE_TAX_VALUE=0
 EIMS_INCOME_WITHHOLD_VALUE=0
@@ -187,6 +189,9 @@ EIMS_PAYMENT_TERM=IMMIDIATE
 EIMS_UNIT_DEFAULT=PCS
 # MoR numeric country code for the buyer; our companies store the country name.
 EIMS_BUYER_COUNTRY_CODE=
+# Buyer region name -> MoR numeric code. companies.region holds names; MoR wants ^[0-9]{1,3}$.
+# An unmapped region fails locally rather than being filed with a guess.
+EIMS_BUYER_REGION_CODES=Addis Ababa=13
 EIMS_CASHIER_NAME=
 EIMS_SALESPERSON_NAME=
 # Automatic filing of issued invoices (@Cron sweep, one invoice per tick).