merge: P4b S5, sent tracking (branch S5, ad649be3)

The second of the paused branches. invoice.sent, Ledger::mark_sent (with
its Lean step), books.mark_sent in its own group, the two attention
items, schema 17. Conflicts, resolved as plan 8.0.2 says: the shared
over-the-wire test file crates/ledger-cli/tests/documents.rs (S4.4's
file kept whole, S5's test appended, its create call given the
confirmed: true the S4.4 contract now needs), types.rs (both appends
kept), docs/parity.md (the review's oracle row and S5's new row).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HS8WZziDnyxunC23MUxrLp

docs/spec/rules.md

@@ -1746,8 +1746,11 @@ person, a billing or shipping address, a billing email (D-55); an
 `invoice.created` or `bill.created`
 carrying a number, terms, a bill-to or seller snapshot, a customer
 reference or memo, a vendor reference, a receipt date or a duplicate's
-reason, or a line naming its account, quantity or unit price (S4.3); and
-S5 to S7's sends, settlements and write-offs. An older version would
+reason, or a line naming its account, quantity or unit price (S4.3);
+S5's `invoice.sent` (who sent an invoice, how, to whom and when: a record
+that moves no figure); S6's payments with `line`, `discount`,
+`discount_line`, `discount_reason`, `fee` or `fee_line`; and S7's
+`invoice.written_off`. An older version would
 restore the document without them silently: every invoice line would post
 to the revenue role and every bill line to general expenses, and every new
 event type would be refused. A command asks for