viewer: no false "database differs" while the books are being written

New concurrency test: one thread writes entries, drafts and receipts as
fast as it can while a Snapshot (the viewer) refreshes in a loop. At
every refresh the snapshot's books must balance, never go backward, and
never see the writer's database as differing from the journal; at the
end it must equal the writer in full. It failed 30 runs in 50:
check_disk read the applied position and the tables in separate
statements, so a write landing between them made a consistent database
look like it "differs", which the viewer shows as a problem in Needs
attention.

- check_disk reads in one read transaction (one consistent view).
- A database that has applied lines the viewer hasn't read yet is now
  DiskCheck::Ahead ("viewer catching up"), not Behind.
- Without the transaction the test fails 14 runs in 20; with it, 50/50.

Also: members' equity spec 3b states the exact bound (a member's period
income is within two cents of their share, one cent of rounding at each
end; the running balance chains exactly), and the spec-blind property
checks that bound. docs/testing.md gains the concurrency layer and item
12: each close into members' capital still rounds on its own (pending).

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

crates/ledger-core/src/reports.rs

@@ -832,8 +832,10 @@ pub struct MembersEquityStatement {
 ///      1065 Schedule K-1 instructions, Item L, "beginning capital account
 ///      and ending capital account ... based on the partnership's books").
 ///   3b. A member's income is the roll-forward's remainder: their share of
-///      the period's income within a rounding cent; the members' incomes
-///      MUST sum to the period's net income.
+///      the period's income, off by less than two cents (under one cent of
+///      rounding at each end of the cumulative split); the members'
+///      incomes MUST sum to the period's net income. (Earnings a close has
+///      already moved into capital were rounded by that close, B-13.)
 ///   4. NEVER hide an unexplained change: it goes in "other".
 ///   5. An entry dated `start` is period activity, not beginning capital.
 /// SCENARIOS: