members' equity: split undistributed earnings cumulatively

A capital account is one running balance (Form 1065 Schedule K-1
instructions, Item L: beginning and ending capital "based on the
partnership's books and records"), but the statement split prior
earnings and the period's income separately, so a member's ending on one
statement could differ by a cent from their beginning on the next.

Now each member's share of undistributed earnings is allocated as of a
date (largest remainder, as before): beginning as of the day before the
period, ending as of its last day. A member's period income is the
roll-forward's remainder (their share within a rounding cent); incomes
still sum to net income. New property
each_members_capital_chains_across_statements (random books, odd-cent
shares, closes to members or not, any split date) fails on the old code
and passes now. The hand-computed test is recomputed: the old split gave
two cents off an even 50/50 to date.

Also: rules.md B-13 quotes Item L; docs/testing.md #7 and #8 marked
changed; the CPA-question list is replaced by the rule that wanderbooks
settles questions from the sources and makes real choices per-company
settings.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GWVo7x3UHw5kwCUaWxiejX

docs/spec/rules.md

@@ -217,10 +217,20 @@ closed period).
 - **No authority** says how to round a share to the cent. Largest
   remainder, ties to the earlier member: DECISION (Stephen, 2026-10-13).
   Closes made before then replay with the last member absorbing it.
+- A capital account is one running balance: Form 1065 Schedule K-1
+  instructions, Item L: "The partnership must report your beginning
+  capital account and ending capital account for the year", "based on the
+  partnership's books and records." So a statement of members' equity
+  splits undistributed earnings cumulatively, as of each date (beginning
+  as of the day before the period, ending as of its last day), never one
+  period at a time: a member's ending on one statement is their beginning
+  on the next, to the cent, and a member's period income is the
+  roll-forward's remainder (their share within a rounding cent).
 
 **Enforced:** `CloseTo::Members`, `periods::allocate`,
 `kernel::apportion`, `members_equity`. **Checked:** Kani
-`apportion_is_exact`, `apportion_is_fair`; properties `prop_split_*`;
+`apportion_is_exact`, `apportion_is_fair`; properties `prop_split_*`,
+`each_members_capital_chains_across_statements`;
 `closes_journaled_under_the_old_rule_replay_with_it`.
 
 ## B-14 Statements: P&L for a period; cash flow by the indirect method