fix(balances): materialize entries dated before the opening anchor - #2434

Problem

Manual Valuation/reconciliation entries dated before an account's opening_anchor are stored but never materialized into Balance rows, so the net-worth chart is empty for that period and a normal sync never backfills it.

Root cause: both balance calculators use account.opening_anchor_date as the hard lower bound of iteration:

• Balance::ForwardCalculator#calc_start_date → account.opening_anchor_date
• Balance::ReverseCalculator → current_anchor_date.downto(account.opening_anchor_date)

and Account::OpeningBalanceManager#opening_date returns the opening-anchor valuation's date whenever one exists. The system intends the opening anchor to sit at/before the oldest entry (set_opening_balance even validates this), but the reconciliation path (create_reconciliation) does not move the anchor back when a valuation is created with an earlier date. The invariant breaks, and every entry before the anchor is silently clipped out of the series.

How to reproduce

1. Create an account (gets an opening_anchor at, say, today).
2. Backfill a reconciliation Valuation dated a year ago.
3. Sync. The valuation exists, but account.balances has no row for that date → net-worth chart flat/empty before the anchor.

Fix

Bound the calculation window on min(opening_anchor_date, oldest_entry_date) instead of the anchor date alone (new shared Balance::BaseCalculator#calculation_start_date).

• Forward: when the window starts before the anchor, seed the running balance from zero. The earliest reconciliation and the opening anchor each reset the absolute balance on their own dates via the existing valuation-override path, so only the pre-anchor opening day's *_adjustments field is affected — later totals are correct.
• Reverse: extend the downto bound; use_opening_anchor_for_date? still keys off the anchor's real date, so the anchor's own treatment is unchanged and pre-anchor reconciliation waypoints reset normally.

No data migration and no anchor mutation — purely a calculation-window fix, so it heals existing accounts on the next sync.

Tests

• forward_calculator_test.rb: "materializes entries dated before the opening anchor" — reconciliation before the anchor is materialized; window starts at the earliest entry; balances correct across the anchor.
• reverse_calculator_test.rb: "materializes reconciliation waypoints dated before the opening anchor".

Summary by CodeRabbit

• Bug Fixes
  • Improved balance materialization to correctly handle ledger entries and reconciliation waypoints that occur before an account’s opening anchor.
  • Updated forward, reverse, and incremental sync behavior to use the calculator’s effective lower-bound start date, ensuring correct initialization, resets, carry-forward, and preventing incremental purges from deleting pre-anchor balances.
• Tests
  • Added regression coverage for forward calculation seeding before the opening anchor.
  • Added regression coverage for reverse calculation materializing pre-anchor reconciliation waypoints.
  • Added regression coverage ensuring incremental sync preserves pre-anchor balances.