sync: shared notes in the feed, revocations, and text pushes from an edit share

The change feed answers `?shares=1` with every note the caller can see, each
saying how it is held, plus a `revoked` list of notes that left them. Granting
a share moves the note past the recipient's cursor; ending one, or the owner
deleting the note, leaves a revocation on the same cursor. A recipient at edit
may push the note's text, and nothing else. Protocol 6, feature `shares`;
opt-in, so the floor stays at 3.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_sync.py

@@ -68,32 +68,39 @@ def test_clamp_limit():
 
 def test_page_cursor_all_drained():
     # Neither stream is full → cursor is the max revision seen; nothing more to page.
-    cursor, more = _page_cursor([1, 3, 5], [2, 4], since=0, limit=500)
+    cursor, more = _page_cursor([[1, 3, 5], [2, 4], []], since=0, limit=500)
     assert cursor == 5
     assert more is False
 
 
 def test_page_cursor_empty():
     # No changes since the cursor → cursor stays put, no more pages.
-    cursor, more = _page_cursor([], [], since=7, limit=500)
+    cursor, more = _page_cursor([[], [], []], since=7, limit=500)
     assert cursor == 7
     assert more is False
 
 
 def test_page_cursor_one_stream_full_advances_to_its_boundary():
     # Notes came back full (limit=3) → truncate at its boundary; later labels defer.
-    cursor, more = _page_cursor([1, 2, 3], [4, 5], since=0, limit=3)
+    cursor, more = _page_cursor([[1, 2, 3], [4, 5], []], since=0, limit=3)
     assert cursor == 3
     assert more is True
 
 
 def test_page_cursor_both_full_uses_min_boundary():
     # Both full → advance only to the SMALLER boundary so neither stream skips a gap.
-    cursor, more = _page_cursor([1, 2, 10], [3, 4, 5], since=0, limit=3)
+    cursor, more = _page_cursor([[1, 2, 10], [3, 4, 5], []], since=0, limit=3)
     assert cursor == 5
     assert more is True
 
 
+def test_page_cursor_a_full_revocation_stream_holds_the_cursor_back():
+    # Revocations page on the same cursor: a full one truncates like any other stream.
+    cursor, more = _page_cursor([[1, 9], [], [2, 3, 4]], since=0, limit=3)
+    assert cursor == 4
+    assert more is True
+
+
 # --- protocol handshake (M10.6) ---------------------------------------------