feat(mcp): books.mark_sent, and the send in the read tools (P4b S5)

`books.mark_sent` records the send the human or a delegate made: the
invoice (`invoice:7` or its id), the day, who sent it (the token's name
unless given), how (email by default, or mail, portal, hand, another
channel), to whom (the bill-to email, else the customer's), whether it
was the original or a reminder (original on the first send, reminder
after), and the channel's own id. It answers with every send on the
invoice, newest last. `books.documents` rows gain `last_sent` and
`books.document` shows the sends, so "did we send it, and when" is one
read away, while a bill (which we receive) has none.

The schema lint refuses a parameter named `to` (a path-name rule), so
the recipients parameter is `recipients`; the journal event and the
result keep the plan's `to`. The tool needs `books:write`: it changes
the record, never a figure.

The handbook gains the invoices-and-bills section (S4.5 had not landed
yet) with the send procedure: the books never send, the PDF goes to the
human, and each send is recorded afterwards.

Over the wire, mark_sent_records_who_and_how: two sends (the second a
reminder with a message id), the list's `last_sent`, the document's
`sends`, and the invoice still owing its total. The P2 cutover refuses
a send before consent and takes it after.

crates/ledger-cli/tests/format_upgrade.rs

@@ -703,6 +703,16 @@ fn books_p2_served_cut_over_whole() {
     // A project by name alone is a plain class, which P2 reads.
     a.call("books.add_project", json!({"name": "Lighthouse"}))
         .unwrap();
+    // An invoice for the send below: a document P2 reads (no new fields),
+    // so it is written before the go-ahead; its send waits for it.
+    let invoice = a
+        .call(
+            "books.create_invoice",
+            json!({"party_id": 1, "date": "2026-01-05", "due_date": "2026-02-04",
+                   "lines": [{"description": "Design", "amount": "400.00"}]}),
+        )
+        .unwrap()["id"]
+        .clone();
     let waiting = journal(&dir);
     let refused: Vec<(&str, Value)> = vec![
         // An invoice's or bill's line naming its project
@@ -729,6 +739,13 @@ fn books_p2_served_cut_over_whole() {
             json!({"decided_by": "Avery Quill",
                    "changes": [{"key": "invoicing.default_terms", "value": "Net 30"}]}),
         ),
+        // A recorded send (invoice.sent, documents_v2, P4b S5): an older
+        // version refuses the type.
+        (
+            "books.mark_sent",
+            json!({"invoice": invoice.to_string(), "on": "2026-01-06",
+                   "by": "Avery Quill", "recipients": ["billing@fernhill.example"]}),
+        ),
         (
             "books.add_account",
             json!({"code": "4900", "name": "Gain on sale", "account_type": "revenue",