Add account balance anchors

OPERATIONS.txt

@@ -649,7 +649,20 @@ exists to be compared with the figures a bank or broker shows on its own screen.
 
 A cash balance equals the real balance only when the journal holds that
 account's complete history. A broker export does; a date-windowed bank statement
-does not.
+does not. A connected cash account closes that gap with a balance anchor: after
+its first successful sync, the bank's booked (CLBD) balance is captured once,
+verbatim, with the day it was true, and stored on the account (anchor_balance,
+anchor_date in accounts.finance). The start balance - the money from before the
+recorded rows - is derived as the anchor less every movement booked through the
+anchor day, and reads as the first line of the account's flow breakdown. Because
+the bank's figure is stored rather than the derivation, importing older history
+later corrects the start balance by itself. An available or expected balance is
+never anchored: it includes pending amounts with no booked fact to subtract. The
+anchor is set once and never moved by later syncs; clear it in the account's
+edit form and the next successful sync captures a fresh one. Running-balance
+checks are only judged after the anchor day, where the balance is observable.
+Anchors are refused on investment accounts, whose broker exports carry their
+complete history.
 
 Checks that fail mean the journal disagrees with itself: row arithmetic, cash
 never negative, holdings never negative. A negative holding means a position was