Fix #776: the Pen continues any open path and joins two paths

As in Illustrator, the Pen shows its continue cursor over an end of any
open path, selected or not, and a click there selects the path and draws
on from that end (reversing it from its first end). While drawing, it
shows a join cursor over an end of another open path, and a click joins
the two into one path, in one undo step, which finishes the path.

path.join takes {ids: [a, b], ends: ["first"|"last", "first"|"last"]}
to join the ends asked for (the Pen's join); without them it joins the
selected paths' nearest ends as before. Joining a path with itself, or
anything that isn't a path, is an error.

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

docs/mcp.md

@@ -1150,7 +1150,8 @@ or pulls handles out in line with the neighbouring anchors (a smooth anchor keep
 (Cut Path at Selected Anchor Points) cuts there and answers `{ids}`: a closed path opens at the cut, its two ends
 on top of each other, and an open path becomes one path per piece. Each cut leaves one of its two anchors selected,
 so `path.moveAnchors {dx, dy}` (or a Direct Selection drag) pulls the path apart there; `path.join {}` (Connect
-Selected End Points) joins the ends again. `path.convertAnchor {id, subpath?, anchor, to, x?, y?}` and
+Selected End Points) joins the ends again; `path.join {ids: [a, b], ends: ["last", "first"]}` joins the ends asked for
+(the Pen's join, #776: drawing on from one open path, a click on another's end makes them one path). `path.convertAnchor {id, subpath?, anchor, to, x?, y?}` and
 `path.split {id, subpath?, anchor}` do the same to one anchor (the Anchor Point and Scissors tools); the Pen with Alt
 held over a selected path's handle, anchor or segment works as the Anchor Point tool. `path.reshapeSegment {id,
 subpath?, segment, t, dx, dy}` (a segment dragged with Direct Selection or the Anchor Point tool) moves the segment's