Cosmos: aurora cursor, card tilt, comets, ripples, turtle; headless sections

Pushed the cosmos theme further:

- Cursor aurora — 600px soft cyan/violet/magenta halo follows the
  pointer with lag (mix-blend-mode: screen, blur 8px). Fades in on
  first move, fades out on pointerleave.
- Card 3D tilt — every cosmos card rotates up to ~6deg toward the
  cursor on hover via custom-property-driven rotateX/rotateY, with a
  cursor-tracking radial shimmer painted inside via mix-blend-mode:
  screen. RAF-throttled. Skipped for testimonial + client tiles since
  they have no chrome.
- Hero parallax — name, tagline, and drift-orb all shift relative to
  cursor position via --cm-x / --cm-y custom properties set globally
  from the pointer move handler.
- Shader: doubled the gravity-well strength (0.03 -> 0.06) and added
  a click-driven ripple — pointerdown sets u_ripple_pos/u_ripple_age
  uniforms; the shader propagates a cyan/magenta luminous ring of
  displacement that decays over ~2s.
- Comets — 3 CSS-only streaks crossing the page on staggered 14/22/
  19s loops with cyan/violet drop-shadow trails.
- Turtle — new .cosmos-turtle img auto-loads if assets/img/turtle.png
  resolves (or theme.turtle URL is set). Floats in from the left,
  arcs across, fades out off the right; pulses its aurora glow on a
  4s bob. Reduced-motion users see it positioned statically.
- All new chrome respects prefers-reduced-motion: halo/comets hidden,
  card tilt + parallax disabled, turtle pinned static.

Also adds section.headless = true (per user request): renders the
section body with no header/kicker chrome and a tighter top margin so
the contents read as nested under the prior section. Example JSON now
uses this to slide the testimonials carousel under the clients wall.

data/links.example.json

@@ -96,6 +96,7 @@
       "label": "Testimonials",
       "kicker": "What people say",
       "layout": "testimonials",
+      "headless": true,
       "items": [
         {
           "type": "testimonial",