refactor(settings): the slice's contracts and notes (P4b S4.2)

invoicing.first_number's refusal (LedgerError::FirstNumberLocked) lands
as the contract the store's check raises once invoices carry numbers
(S4.3): a series that has begun is never renumbered (B-40, A-2).

Also: the settings invariants name the number prefix's bound, update_party's
doc says which facts wait for documents_v2, and the email check no longer
reads as a single-arm match. Terms is S4.1's as it landed; this slice no
longer carries a copy.

crates/ledger-core/src/ledger.rs

@@ -4702,12 +4702,11 @@ impl Ledger {
                 }
             }
             Archived(true) => {}
-            Email(v) => match v {
-                Some(e) => {
-                    crate::parties::check_email("email", e).map_err(LedgerError::InvalidParty)?
+            Email(v) => {
+                if let Some(e) = v {
+                    crate::parties::check_email("email", e).map_err(LedgerError::InvalidParty)?;
                 }
-                None => {}
-            },
+            }
             // Terms are typed and already canonical (`crate::Terms`), so
             // there is nothing here a string could get wrong.
             DefaultTermsAsCustomer(_) | DefaultTermsAsVendor(_) => {}