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

crates/ledger-core/tests/statements.proptest-regressions

@@ -0,0 +1,7 @@
+# Seeds for failure cases proptest has generated in the past. It is
+# automatically read and these particular cases re-run before any
+# novel cases are generated.
+#
+# It is recommended to check this file in to source control so that
+# everyone who runs the test benefits from these saved cases.
+cc b177aed2b1d69b6a0abb37b170ec5b9efc73d7fae49bace065c15c819ecca711 # shrinks to months = [(0, 0, 0, 507, 37392), (0, 0, 0, 9054, 3952), (0, 0, 0, 34144, 7979)], closes = [false, true, false, false, false, false, false, false, false, false, false, false], split = 14