content(media): replace temporary generated concept art with human-created visuals

[zoeyrose]

Outcome

Replace the four temporary OpenAI-generated website concept images with
human-created Atrinik visuals so the public site itself fully embodies the
project's message: human-made world and art, agent-driven software.

The current images are transparently credited, but they are temporary and are
the sole known exception to the intended “generative AI is used for code, not
creative content” direction.

Current scope

The four records in src/data/media.json supply the home hero, three chronicle
panels, and the global social card. Their sources and credits are also recorded
in artwork/originals/, PROVENANCE.md, THIRD_PARTY_NOTICES.md, and
artwork/README.md.

Scope

• Replace every published/generated concept image and social-card use with
  provenance-cleared work made by a human creator.
• Prefer authentic Atrinik pixel art, world art, or representative gameplay
  imagery when its source, permission, attribution, and presentation context
  are proven. If suitable media is not ready, use an honest typography/CSS
  treatment rather than another generated placeholder.
• Do not present concept work as gameplay footage or promise unimplemented
  features.
• Update src/data/media.json, original/derivative storage, accessible text,
  authorship, licenses, transformations, notices, and source hashes together.
• Remove the generated files from the current tree without rewriting Git
  history. Update provenance/notice prose so the historical replacement is
  transparent rather than silently erasing the former source.
• Recheck page weight, responsive crops, contrast, semantic alternatives, and
  social-preview output.

Acceptance criteria

• No OpenAI- or other generative-AI-created bitmap is published by the
  built site or used as its social preview.
• Every replacement has a retained local source and immutable provenance,
  human creator, exact license, transformation, digest, dimensions, notice,
  and accessible alternative text/role.
• Home/chronicle copy accurately describes what each visual depicts and
  does not imply unavailable gameplay.
• Removed generated artwork remains recoverable from ordinary Git history;
  no history rewrite occurs.
• npm run check, npm run build, npm run deploy:dry-run, and
  git diff --check pass, followed by keyboard/screen-reader and
  mobile/desktop review.

Coordinate with the public authorship-disclosure work and the existing website
information-architecture roadmap in #4.