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/src/reports.rs

@@ -458,7 +458,7 @@ fn statement_lines(
 /// SCENARIOS:
 ///   1. IF a month was closed: SHOULD still report that month's earnings.
 ///      BECAUSE closing doesn't make a period break even (R-9).
-///   2. IF `start` is after `end`: SHOULD refuse. (ASSUMED)
+///   2. IF `start` is after `end`: SHOULD refuse. (DESIGN: no standard governs a reversed range; docs/testing.md #1)
 /// ```
 pub fn profit_and_loss_for_period(
     ledger: &Ledger,