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-gui/src/app.rs

@@ -700,6 +700,7 @@ fn checks(b: &Books) -> Vec<(bool, String, String)> {
     out.push(match &b.integrity.disk {
         DiskCheck::Matches => (true, "Database matches journal".to_string(), "The server's SQLite database matches a fresh projection of the journal, every table, row for row.".to_string()),
         DiskCheck::Behind { applied, journal } => (true, "Database catching up".to_string(), format!("The database has applied {applied} of {journal} journal events; the server applies the rest right after writing them.")),
+        DiskCheck::Ahead { .. } => (true, "Viewer catching up".to_string(), "The server has written journal events this view hasn't read yet; it reads them on its next refresh.".to_string()),
         DiskCheck::Missing => (true, "Read-only copy".to_string(), "No server database here: a copy of the books (a CPA's, say), or the server hasn't run yet. The journal and every file are verified either way.".to_string()),
         DiskCheck::Differs(why) => (false, "Database differs".to_string(), format!("{why}. The server rebuilds it from the journal (books.verify, or on its next start).")),
         DiskCheck::Unreadable(why) => (false, "Database unreadable".to_string(), why.clone()),