brand: add Canva symbolic hero + feature on README (#326)

## Summary
Follow-up to the merged brand pack (#325). Adds the generated **symbolic
hero** you selected (Canva Candidate 2) and features it on the README.

- `brand/hero/social-profile.png` — Linear-Amber symbolic hero: the
"exactly once" amber thread converging to a single luminous point on the
horizon (the resource, consumed once).
- README centerpiece now uses this hero; the wordmark
`social-preview.svg` remains the brand banner / GitHub social-preview
asset.

Pre-existing governance CI (licence-consistency = owner-only
LICENSE/Cargo.toml SPDX, Hypatia, Trusted-base) is unrelated to this
additive change.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

https://claude.ai/code/session_01L7ysyzRpm8y7cqJpBLxwKi

---
_Generated by [Claude
Code](https://claude.ai/code/session_01L7ysyzRpm8y7cqJpBLxwKi)_

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

Author: hyperpolymath

README.adoc

@@ -4,7 +4,7 @@ image:https://img.shields.io/badge/Philosophy-Palimpsest-indigo.svg[Palimpsest,l
 image:https://github.com/hyperpolymath/ephapax/actions/workflows/codeql.yml/badge.svg?branch=main[CodeQL,link="https://github.com/hyperpolymath/ephapax/actions/workflows/codeql.yml"]
 image:https://api.thegreenwebfoundation.org/greencheckimage/ephapax.org[Green Hosting,link="https://www.thegreenwebfoundation.org/green-web-check/?url=ephapax.org"]
 
-image::brand/social-preview.svg[ephapax,align="center"]
+image::brand/hero/social-profile.png[ephapax,align="center",width=460]
 
 // SPDX-License-Identifier: CC-BY-SA-4.0
 // Owner: Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>