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_ledger.rs

@@ -1235,7 +1235,7 @@ fn a_loss_closes_as_a_debit_to_equity() {
 }
 
 /// Oracle: close INVARIANT 5: nothing to close posts nothing and returns
-/// None. ASSUMED: the period is still marked closed (step 3 of the doc).
+/// None. CONVENTION (docs/testing.md #4): the period is still marked closed (step 3 of the doc).
 #[test]
 fn a_close_with_nothing_to_close_posts_no_entry() {
     let mut b = books();