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

crates/ledger-mcp/handbook/invoices-and-bills.md

@@ -0,0 +1,28 @@
+# Invoices and bills
+
+An invoice bills a customer: it posts Dr Accounts Receivable and Cr the
+revenue its lines name. A bill is a vendor's invoice to us: it posts Dr
+the expense or asset its lines name, and Cr Accounts Payable. A document's
+own figures are read from the ledger lines tagged with it, never kept on
+the document: a reversed payment, a void or a write-off comes out right
+without another copy to correct (B-38).
+
+## Sending an invoice
+
+- The books never send mail, and never say they did. To send an invoice,
+  create and post it (`books.create_invoice`), hand its PDF to the human
+  (`books.document` with `pdf: true`), and let the owner or a delegate
+  send it.
+- Then record the send with `books.mark_sent`: the invoice (`invoice:7`),
+  the day it went out, who sent it (`by`: the owner, or the agent acting
+  for them), how (`via`: `email`, or `mail`, `portal`, `hand`, or another
+  channel's own name), to whom (`to`: the bill-to email, else the
+  customer's email), whether it was the original or a reminder (`kind`:
+  `original` for the first send, `reminder` after), and the channel's own
+  reference when there is one (a message id).
+- A send changes no figure and never moves the due date. Several sends
+  are allowed: each is recorded in order, and `books.documents` shows the
+  latest as `last_sent`.
+- A void invoice can't be marked sent: it should never have reached the
+  customer. An invoice already open, partly paid, paid or written off can
+  (a copy for the customer's records, a reminder).