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/bookkeeping/dimension-rules.ts

@@ -186,7 +186,13 @@ export function assertMandatoryDimensions(
  *     result account must not be able to block closing the year
  *   - storno/correction/credit notes are HOW history gets fixed — blocking
  *     them on entries that pre-date a rule would make old mistakes
- *     permanent (same argument as the commitEntry bypass for reversals)
+ *     permanent (same argument as the commitEntry bypass for reversals).
+ *     Credit notes specifically COPY the original's bags (PR7) so the
+ *     reversal nets against the same dimension cells: if the original
+ *     satisfied the rules, so does the copy (enforcement = no-op); if the
+ *     original pre-dates the rules, enforcing would demand an ASYMMETRIC
+ *     tag — a credit in P001 with no original in P001 — which is exactly
+ *     the project-P&L skew this feature exists to prevent
  *   - accrual dissolutions replay a schedule created before the rule
  *
  * Operational sources (manual, bank_transaction, invoice_*, supplier_*

lib/bookkeeping/engine.ts

@@ -592,6 +592,9 @@ export async function commitEntry(
       // System/correction sources are exempt — see
       // DIMENSION_RULE_EXEMPT_SOURCE_TYPES (imported history, bokslut
       // mechanics and credit instruments must never be blocked by policy).
+      // source_type is a HEADER column (journal_entries) — the join repeats
+      // the same value on every line, so reading lines[0] IS reading the
+      // entry header; lines cannot mix source types.
       if (!isDimensionRuleExemptSource(typedLines[0]?.journal_entries?.source_type)) {
         assertMandatoryDimensions(typedLines, rules)
       }

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', () => {