docs(dimensions): post-merge review nits from #888 (#889)

- SIE round-trip fixture: vouchers in ascending verno order (A1, A2, A3)
  per SIE4 core invariant 5 — the custom-dim voucher was spliced out of
  order
- commitEntry: note that source_type is a HEADER column repeated by the
  join — reading lines[0] IS reading the entry header, lines cannot mix
  source types (a reviewer misread this as per-line logic)
- dimension-rules: sharpen the credit-note exemption rationale — credit
  notes copy the original's bags, so enforcement is either a no-op or
  would force the exact asymmetric-tag P&L skew the feature prevents

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

lib/import/__tests__/sie-dimensions-roundtrip.test.ts

@@ -69,11 +69,6 @@ const SOURCE_SIE = [
   '#TRANS 5010 {1 "KS01" 2 "KB1" 6 "P001"} 15000.00',
   '#TRANS 1930 {} -15000.00',
   '}',
-  '#VER A 3 20260117 "Avdelningskostnad"',
-  '{',
-  '#TRANS 5010 {20 "SYD" 25 "T1"} 800.00',
-  '#TRANS 1930 {} -800.00',
-  '}',
   '#VER A 2 20260116 "Odeklarerat projekt"',
   '{',
   // P002 is referenced but never declared via #OBJEKT — import synthesizes
@@ -81,6 +76,11 @@ const SOURCE_SIE = [
   '#TRANS 5010 {6 "P002"} 500.00',
   '#TRANS 1930 {} -500.00',
   '}',
+  '#VER A 3 20260117 "Avdelningskostnad"',
+  '{',
+  '#TRANS 5010 {20 "SYD" 25 "T1"} 800.00',
+  '#TRANS 1930 {} -800.00',
+  '}',
 ].join('\n')
 
 describe('SIE dimensions round-trip', () => {