docs: sent tracking in the rules, parity and test records (P4b S5)

rules.md B-27 lists `documents_v2`, the one feature S4 to S7 share
(D-52), including `invoice.sent`; receipt-store.md lists the event;
parity.md gains the sent-tracking row (done: implemented, tested, and
golden-covered by the trial balance of the fixture book around a send);
testing.md records the tests and what P12 must mutate, and names the
two attention predicates as deferred with step 4 until U0b lands.

docs/testing.md

@@ -3050,3 +3050,54 @@ and `BooksServer::new`'s copy (a dropped field must fail
 `isolation_label_is_enforced_under_the_unit`); the host's `server_flags`
 extension and `parse_args`' passthrough of `--isolation` and its optional
 `--http` value.
+
+## P4b S5: sent tracking (2026-10-27)
+
+The books record that an invoice went out; they never send it (A-4). A
+send is `invoice.sent`, under `documents_v2` (D-52), and moves no figure:
+`Ledger::mark_sent` pushes a `Sent` onto the invoice, the store journals
+one line and no entry, replay restores it and the projection keeps it in
+`schema` v17's `invoice_sends` (S4.3 took 16).
+
+- **`ledger-store` `tests/arap_standing.rs`:** `mark_sent_waits_for_documents_v2`
+  (books with no features: refused, nothing journaled; after `upgrade`,
+  taken, and the reopen equals what was live), `sends_replay_and_project`
+  (two sends: the journal's `invoice.sent` carries the whole record in
+  order, `verify` passes, the reopened invoice holds both),
+  `a_send_changes_no_figure` (on the `p4-arap-before` fixture: the report
+  snapshot, the invoice's `DocState` and every derived cache are equal
+  before and after), `mark_sent_on_void_refused` (`DocumentVoid`,
+  nothing journaled; a send dated before the void is taken).
+- **`ledger-store` `tests/arap_props.rs`:** `status_is_a_function_of_journal_and_date`
+  gains `Op::Sent` (random sends on invoices, skipped on bills): every
+  figure, the control check and both agings still agree across live,
+  replay and snapshot.
+- **`ledger-store` `tests/format.rs`:** `an_older_version_stops_at_invoice_sent`
+  (`invoice.sent` is this version's, a reader without `documents_v2` waits,
+  and the books declare the feature before the first send line).
+- **8.0.6:** `books_written_under_older_rules_still_open` hand-writes an
+  `invoice.sent` and still opens; the fixture `old-books/p4_s5_before`
+  was written by eeee9658 (the revision before this unit) and opens to
+  the same figures; `books_p2_served_cut_over_whole` refuses a send before
+  consent and takes it after.
+- **`ledger-cli` `tests/documents.rs`:** `mark_sent_records_who_and_how`
+  (two sends over the wire: the token's name, email and original by
+  default, then a reminder with a message id; `books.documents` shows
+  `last_sent`, `books.document` both sends, and the invoice still owes its
+  total).
+- **`ledger-mcp/src/handbook.rs`:** the new `invoices-and-bills` section
+  renders, and its id is unique (the existing handbook tests).
+
+**For P12 (mutation targets):** `Ledger::mark_sent`'s refusals (unknown
+invoice, a send dated before it, `DocumentVoid`, empty `to`, and each of
+the three `check_text` calls; the note's check is stricter than the plan's
+list, to keep a full TIN out of the journal); `SentVia::parse`'s arms and
+its `other:` prefix; `SendKind::parse`'s two arms; `sent_via`'s arms and
+its empty fallback; `send_kind`'s error arm; the handler's `kind` default
+(`original` when `sends` is empty, `reminder` after), its `to` fallback
+(bill-to, then party, then refused) and its `by` fallback; `invoice_id`'s
+`invoice:` strip; `last_sent`'s `max`; and the replay reader's
+`newer_content` arms for an unknown channel or kind. The two attention
+predicates ("invoice never sent", "overdue invoice not reminded") and
+their day comparisons are deferred with the unit's step 4 until U0b
+lands, and are named here when they do.