fix(bookkeeping): allow a first rakenskapsar shorter than 6 months (#1165)

The validator enforced a 6-month minimum on the FIRST fiscal period,
citing BFL 3 kap. The law says the opposite: BFL 3 kap 3 par expressly
allows a rakenskapsar shorter than 12 months, with no floor, when
bokforingsskyldigheten begins (Bolagsverket: the first year may be
"hur kort som helst", max 18 months). The floor only applied to
isFirstPeriod, exactly the case the law exempts, and blocked an
autumn-registered AB from shortening its first year to Dec 31 to file
an early arsredovisning.

Drop the minimum, keep the 18-month cap and the day-boundary rules,
and remove the now-dead Swedish error mapping.

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

lib/bookkeeping/__tests__/validate-period-duration.test.ts

@@ -84,16 +84,16 @@ describe('validatePeriodDuration', () => {
     expect(result).toContain('18 months')
   })
 
-  it('enforces 6-month minimum for first period (2 months)', () => {
-    expect(validatePeriodDuration('2026-03-25', '2026-05-31', { isFirstPeriod: true })).toBe(
-      'First fiscal period must be at least 6 months (BFL 3 kap.)'
-    )
+  // BFL 3 kap 3 § sets NO minimum for a first räkenskapsår: it may be
+  // "hur kort som helst" (Bolagsverket). A 6-month floor here blocked an
+  // autumn-registered AB from shortening its first year to Dec 31 for an
+  // early årsredovisning (real customer case 2026-07-25).
+  it('allows a short first period (3 months, autumn registration to Dec 31)', () => {
+    expect(validatePeriodDuration('2025-10-01', '2025-12-31', { isFirstPeriod: true })).toBeNull()
   })
 
-  it('enforces 6-month minimum for first period (5 months)', () => {
-    expect(validatePeriodDuration('2026-08-01', '2026-12-31', { isFirstPeriod: true })).toBe(
-      'First fiscal period must be at least 6 months (BFL 3 kap.)'
-    )
+  it('allows a very short first period (2 months, mid-month start)', () => {
+    expect(validatePeriodDuration('2026-03-25', '2026-05-31', { isFirstPeriod: true })).toBeNull()
   })
 
   it('allows exactly 6 months for first period', () => {

lib/bookkeeping/validate-period-duration.ts

@@ -1,7 +1,10 @@
 /**
  * Validates fiscal period duration per BFL 3 kap.
  * Maximum 18 months for any fiscal period (first year may be extended).
- * Normal ongoing periods are 12 months.
+ * Normal ongoing periods are 12 months. There is NO minimum length: BFL
+ * 3 kap 3 § expressly allows a räkenskapsår shorter than 12 months when
+ * bokföringsskyldigheten begins or the year is re-laid, with no floor
+ * (Bolagsverket: the first year may be "hur kort som helst").
  */
 
 /**
@@ -56,16 +59,13 @@ export function validatePeriodDuration(start: string, end: string, options?: Val
     return 'Period end must be the last day of a month'
   }
 
-  // Max 18 months per BFL 3 kap.
+  // Max 18 months per BFL 3 kap. No minimum: a first (or re-laid) year may
+  // be arbitrarily short, e.g. an autumn-registered AB shortening its first
+  // year to end at Dec 31 for an early årsredovisning.
   const months = monthsBetween(start, end)
   if (months > 18) {
     return `Period duration ${months} months exceeds maximum 18 months (BFL 3 kap.)`
   }
 
-  // First fiscal period must be at least 6 months per BFL 3 kap.
-  if (options?.isFirstPeriod && months < 6) {
-    return `First fiscal period must be at least 6 months (BFL 3 kap.)`
-  }
-
   return null
 }

lib/company/__tests__/compute-fiscal-period.test.ts

@@ -89,14 +89,19 @@ describe('computeFiscalPeriod', () => {
     expect(result.periodName).toBe('')
   })
 
-  it('rejects a first year shorter than 6 months', () => {
+  // BFL 3 kap 3 § allows a first räkenskapsår of any length up to 18 months:
+  // an autumn-registered AB may end its first year at Dec 31 (Bolagsverket
+  // offers this explicitly on the registration certificate).
+  it('accepts a first year shorter than 6 months', () => {
     const result = computeFiscalPeriod({
       entity_type: 'aktiebolag',
       is_first_fiscal_year: true,
       first_year_start: '2026-10-01',
       first_year_end: '2026-12-31',
     })
-    expect(result.error).toContain('at least 6 months')
+    expect(result.error).toBeNull()
+    expect(result.startStr).toBe('2026-10-01')
+    expect(result.endStr).toBe('2026-12-31')
   })
 
   it('rejects an end date that is not the last day of a month', () => {