feat(docs): draw the wiring lane and the honesty lane

[HarperZ9]

What this adds

Two diagrams and a repository mark on the plexus front page, rendered from a spec
rather than drawn by hand.

file	what it shows
docs/art/wiring-lane.svg	manifest, discover, capability, wiring, goal, plan, receipt, verify, ending in still holds or drifted
docs/art/honesty-lane.svg	tool set, emits, consumes, match, leftover, loop, collision, report, ending in wired, colliding, or unmet
docs/art/plexus-header.svg	the repository mark, seeded from the name so no two repositories share one

The honesty surface is the part hardest to picture from prose. Unmet inputs,
unconsumed outputs, feedback loops, and colliding organ ids read as four separate
bullet points, when they are four ways one comparison comes out.

The picture is derived, and a gate says so

docs/art/plexus.art.json is the source. tools/render_repo_art.py renders it;
tools/check_repo_art.py re-renders and compares bytes. Twelve gates run,
covering determinism, per-repository identity, wrapper truncation, return-edge
geometry, outcome-box fit, tagline width, no local paths or em-dashes, and every
committed illustration actually being shown somewhere.

tests/test_repo_art.py runs that gate under pytest and asserts on its receipt,
so python -m pytest -q covers the front page with no CI change. One of the
three tests feeds the outcome-box check a note too wide for its box and requires
a complaint, so a green suite means the gate can still fail.

Two mechanisms moved from the source into the text

Stage labels were written from src/plexus/mesh.py, plan.py, and receipt.py
rather than from the README, which surfaced two things the code does and the front
page did not say:

• verify_plan rebuilds the receipt from the plan it just re-derived, not from
  the body saved in the file, so editing that body cannot make a plan agree with
  itself.
• The receipt's method_version has to match before anything else is compared, so
  a plan written by an older plexus is reported as failing rather than silently
  re-interpreted under new rules.

Line endings

.gitattributes pins docs/art/* to LF. The gate compares committed bytes
against a fresh render, so per-platform normalization would make the comparison
depend on who checked the repository out.

Honest nulls

• The four renderer scripts under tools/ are a copy of the same files in the
  sibling repositories, now the eleventh. That is real duplication and a real
  maintenance cost, taken deliberately so each repository stays dependency-free
  and can render its own artwork from a fresh clone with nothing installed.
• The old .github/assets/zentropy-banner.png is no longer referenced by
  anything. It is left in place rather than deleted, in case something outside
  this repository links to it.
• The diagrams describe the declared mesh. They do not change the standing caveat
  that every edge is self-reported by its producer and that plexus does not import,
  resolve, or run the source a manifest cites.
• Local run: 74 passed, and python examples/tour.py exits 0.
• The artwork has not been viewed as GitHub renders it. Static inspection says it
  will fall back to system fonts, which is inference rather than a look.

🤖 Generated with Claude Code