feat(dimensions): PR9 cutover — cost_center/project become GENERATED columns, dual-write removed (#870)

The dual-write window ends (dev_docs/dimensions_implementation_plan.md PR9):
journal_entry_lines.cost_center/project are now GENERATED ALWAYS AS
(NULLIF(dimensions->>'1'/'6','')) STORED — divergence from the bag is
impossible by construction instead of by convention.

- migration 20260702230000: drift pre-flight (refuses cutover on
  inconsistent data; prod verified 0 drift across 593k rows), column swap
  (DROP metadata-only + one-rewrite ADD pair), and atomic redefinition of
  the two SQL writers — retag_line_dimensions (SET dimensions only) and
  bulk_book_transactions (INSERT names the bag only)
- TS writers stripped of the mirror spread: engine buildLineInserts
  (covers create/update/reversal), storno-service (reversal + correction),
  SIE import bulk insert, sandbox seed
- lineDimensionColumns() removed from dimension-resolver — nothing derives
  mirrors in TypeScript anymore; normalizeLineDimensions + the deprecated
  cost_center/project INPUT aliases stay (API contract, they normalize
  into the bag); JournalEntryLine ROW type keeps the fields (generated
  columns still SELECT)
- immutability carve-out unchanged BY DESIGN: its whole-row diff already
  subtracts dimensions/cost_center/project on both sides, which is exactly
  what makes it correct with generated columns (BEFORE-trigger NEW carries
  not-yet-recomputed mirror values)
- audited every reader (v1 journal-entries, MCP query_journal filters +
  group_by, rc-basis-gaps) — reads are untouched; no index, view, or
  constraint referenced the TEXT columns, so DROP COLUMN cascades nothing
- new pg suite: generated derivation, explicit-mirror-write rejection,
  draft-update recompute; existing retag/substrate/bulk-book suites
  updated to bag-only writes (their mirror assertions now exercise the
  generation expression)

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

lib/core/bookkeeping/storno-service.ts

@@ -6,7 +6,7 @@ import type {
   JournalEntryLine,
 } from '@/types'
 import { validateBalance, getNextVoucherNumber } from '@/lib/bookkeeping/engine'
-import { normalizeLineDimensions, lineDimensionColumns } from '@/lib/bookkeeping/dimension-resolver'
+import { normalizeLineDimensions } from '@/lib/bookkeeping/dimension-resolver'
 import { backfillStandardBASAccounts } from '@/lib/bookkeeping/account-backfill'
 import { resolvePeriodStatusForDate } from '@/lib/core/bookkeeping/period-service'
 import {
@@ -287,7 +287,6 @@ export async function correctEntry(
       line_description: `Storno: ${line.line_description || ''}`,
       tax_code: line.tax_code || null,
       dimensions,
-      ...lineDimensionColumns(dimensions),
       sort_order: index,
     }
   })
@@ -372,7 +371,6 @@ export async function correctEntry(
         line_description: line.line_description || null,
         tax_code: line.tax_code || null,
         dimensions,
-        ...lineDimensionColumns(dimensions),
         sort_order: index,
       }
     })