docs: the assumed list, checked against the standards; mutation score

Each formerly ASSUMED behavior is now CONFIRMED, CONVENTION, DESIGN, or
CHANGE (pending), with its source (Reg. S-X, ASC 230 via Deloitte DART,
IRS K-1 instructions, OpenStax). Two need engine work: members' equity
should allocate cumulatively so each member's beginning equals the prior
ending (#7), and the cash flow should show financing gross and classify
investing by account role (#8). Two questions for the CPA are listed.
The test doc comments that relied on them now cite the verdicts.

Core mutation run: 1,011 caught + 13 timeouts of 1,024 viable, 0 missed.
No benchmark budget, by decision.

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

crates/ledger-core/tests/spec_arap.rs

@@ -538,7 +538,7 @@ fn create_invoice_refuses_due_before_date_books_unchanged() {
 
 /// Closes January 2026 on `l`.
 ///
-/// ASSUMED: closing a period with nothing in it still marks it closed
+/// CONVENTION (docs/testing.md #4): closing a period with nothing in it still marks it closed
 /// (close_period returns Ok(None)); the precondition is asserted.
 fn close_january(l: &mut Ledger) {
     let p = l