docs: regenerate layers diagram — control layer is 'Kernel', not 'ObjectOS' (#2966)

- layers.svg: middle box title ObjectOS → Kernel; heading 'three
  protocol layers' → 'three layers' (matches the merged README wording).
- layers.png: re-rendered from the SVG at the original 1760x821 via
  headless Chromium (Playwright, viewport 1200x560 @ 1.4667x device
  scale). Font rendering uses the Linux fallback of the SVG's system
  font stack; layout and colors are unchanged.

Closes the last visual 'ObjectOS layer' remnant from #2955.


Claude-Session: https://claude.ai/code/session_0162o68e5w3bpUBEVRQboUGG

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

Author: os-zhuang