Sketch on a cylinder primitive's caps; format 15 (#460)

* Sketch on a cylinder primitive's caps (#420)

A solid.cylinder's caps carried no construction certificate, so a sketch
on its top or bottom was refused unless fixed:true (#420's crossed
cylinders, built from primitives).

- kernel: FaceSource::CylinderCap { producer, high, plane };
  Body::with_cylinder_certificate turns a cylinder's two extrude caps into
  certified caps. kernel::cylinder's output is unchanged for its other
  callers.
- doc: FaceCertificate::CylinderCap { high }, named F<id>:top / F<id>:bottom
  as the cylinder's face names already are; New Body and Join cylinders
  are certified, shells carry it as their planar source.
- format 15: designs can store the new certificate kind; version 14
  designs need no change, and older builds ask for an update instead of
  meeting an unknown kind.

Tests: top and bottom attachments follow a height and base edit and a
reopen; crossed joined cylinders keep both caps attachable; a shelled
cylinder's floor keeps its cap identity; malformed certificate JSON is
refused.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YDDPmNdRoBgkHcLeUDKVfb

* Format 15 also covers sketch_axis links (#448)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YDDPmNdRoBgkHcLeUDKVfb

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

docs/book/src/guide/files.md

@@ -51,8 +51,9 @@ keeps the current design. Previews cannot be edited or measured. This experiment
 requires `preview: true` in an engine Save/Save As call; ordinary saves omit display caches.
 The repository's `docs/design-container.md` describes limits and encoding details.
 
-Current designs use format 14, including persistent names from exact face history,
-exact planar shell-face anchors, persistent analytic offset boundary roles and
+Current designs use format 15, including sketch face anchors on cylinder caps, sketch axis
+links, persistent
+names from exact face history, exact planar shell-face anchors, persistent analytic offset boundary roles and
 association profile references, durable sketch face anchors, periodic fit splines and versioned
 revolve conventions. It also retains recorded body transfers, explicit 3D curve coordinate
 spaces and certified face bindings, alongside semantic configuration styles, associative