signatures: a transfer carries one signature per writer in it

Under a policy requiring signatures, a node takes a change only once a signature
vouches for it: one of the change itself, or of a later change of the same writer. A
writer is a node's run, each start, rejoin or drop making a fresh actor, so an epoch
gathers writers for as long as it lasts, and a transfer of its whole history (a joiner
pulling the document, a delta) must carry a signature for each of them, or the joiner
waits for the rest for good.

A message's signatures are now chosen by what it carries:

* changes, up to 256 signed among them: each one's signature;
* more (a whole document pulled): each writer's latest signed change among them, found
  by the document's change metadata (`Signatures::for_changes`), vouching for the rest;
* a delta: each writer with changes in it (`get_changes_meta(from)`), its latest
  verified signature (`for_delta`).

A message carries up to 65536 (6 MiB), and as many wait at the receiver for their
changes (a delta's come after it). So a transfer leaves writers unvouched only in an
epoch with more than 65536 runs writing since it began; a remake starts the count again.
Wire version 48.

1 files changed, 9 insertions, 6 deletions

diff --git a/docs/api.md b/docs/api.md
index 3fb99be..a4b76b0 100644
--- a/docs/api.md
+++ b/docs/api.md
@@ -291,15 +291,18 @@ peer was sent before, so changes sent again (a message lost on the way) arrive
 vouched for as the first time; and a node checks one it receives once it holds the
 change (a signature costs about 30 µs to check, and one vouches for its node's changes
 before it). A message carrying no changes carries no signatures; one carrying more
-than 4096 signed changes (a whole document pulled), and a delta, carry each node run's
-latest instead. A node signs the latest change of each of its runs that nothing vouches
-for as it starts, before anything leaves, and whenever its document is swapped (a drop,
-a switch, a rejoin): changes it kept unsigned, never sent.
+than 256 signed changes (a whole document pulled) carries each writer's latest among
+them instead, and a delta each writer's latest with changes in it. A writer is a node's
+run: each start, rejoin or drop makes a fresh one. A node signs the latest change of
+each of its runs that nothing vouches for as it starts, before anything leaves, and
+whenever its document is swapped (a drop, a switch, a rejoin): changes it kept
+unsigned, never sent.
 
 Two cases still arrive unvouched, and wait under a policy requiring signatures: a change
 vouched for only by a later change of its node that the rules refused (nodes take it,
-then can't pass the proof on, [`future-work.md`](future-work.md)); and, in one transfer,
-the changes of an epoch with more than 4096 node runs writing.
+then can't pass the proof on); and, in one transfer, the changes of runs past the
+65536 a message carries signatures of at most, in an epoch with more runs writing than
+that since it began. A remake starts the count again.
 The signatures a node has are kept in its document file (`--document-file`) with the
 document.