docs(examples): add reproducible screenshots of the sample KBs

[dripsmvcp]

Summary

• the examples/ KBs were text-only — the READMEs told a reader to copy a
  fixture and run vouch status / search / show / audit without ever
  showing what those commands return. this adds screenshots rendered straight
  from the shipped fixtures so the examples carry their own weight.

What's here

• docs/img/examples/render.py — a stdlib-only generator. copies a fixture
  to a throwaway .vouch, runs each documented command with colour disabled,
  normalises the one volatile token (the absolute KB path), and renders the
  output into a self-contained terminal-style SVG.
• docs/img/examples/*.svg — tiny/: status, search auth,
  show prop-001, audit. decision-log/: search free-tier and a
  supersession diff.
• READMEs (examples/README.md, examples/tiny/, examples/decision-log/)
  embed the images next to the commands they show.
• make examples-screenshots regenerates them.
• tests/test_example_screenshots.py asserts the renderer is pure, escapes
  XML, and the committed SVGs are byte-identical to a fresh render.

Decisions / deviations from the issue

• SVG instead of vhs/.tape. the issue suggested a .tape re-rendered
  with vhs, but vhs needs ttyd + ffmpeg. SVG needs only python + the
  vouch CLI, is text (so it diffs in PRs and re-renders byte-for-byte), and
  the test enforces that the committed images stay current — the same
  determinism goal, lighter deps.
• audit-trail shot uses tiny/. the decision-log/ fixture on test
  ships claims only (no audit log), so the audit-trail capture uses tiny/
  (which has a full audit log) and the supersession capture uses
  decision-log/. no fixture is modified, per the issue's compatibility note.
• web review-ui / desktop GUI captures are left as a follow-up — they need
  a browser/Electron runtime to capture deterministically, which is out of
  reach for a text-renderable, CI-checked generator.

Test Plan

• Regression test added: tests/test_example_screenshots.py
• pytest tests/ --ignore=tests/embeddings — green (rc=0)
• ruff check src tests — clean
• mypy src — clean
• images re-render byte-for-byte (make examples-screenshots is idempotent)

Closes #286

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