Cut 5 stale artifacts (plus 1 transitively-stale) + 1 dead test

Per the audit's "next cut" list. Each was a one-time artifact whose
moment has passed:

  docs/markdown-cell-migration-investigation.md
    One-time investigation document for the Markdown migration that
    has long since shipped. Plus the matching
    test_investigation_documents_program_and_cell_solution test in
    tests/test_markdown_migration_prereqs.py (a static-text assertion
    that the investigation doc still says certain phrases — useless
    once the doc is removed).

  docs/shallow-example-audit.md
    One-time audit of which examples were shallow. The findings
    have been folded into example sources and rubrics.

  docs/example-quality-new-vs-old.{md,csv,svg}
    One-time before/after quality comparison from an earlier rubric
    expansion. The improvement arc lives in commit history.

  docs/example-graph-score-impact.md
    Cut transitively — referenced score_examples.py and was itself a
    one-time impact report on the see_also graph rollout.

  public/prototyping/operators-polish-comparison.html
    One-time before/after for the tree-edge alignment fix. Pattern
    is documented in the figure rubric.

  scripts/score_examples.py
    Heuristic scorer superseded by the SCORES dict in
    src/marginalia.py, where every example carries an explicit v2
    rubric score with commentary.

References cleaned up:
  - tests/test_markdown_migration_prereqs.py loses one test method
    and the INVESTIGATION constant.
  - scripts/build_prototypes.py loses the operators-polish-comparison
    entry from the prototyping index.

58 tests pass (was 59; one test removed). SEO/cache lint clears 110
pages. Index.html regenerated without the stale entry.

Net cuts across the two cleanup commits: 8 journey-prototype HTMLs,
1 polish-comparison HTML, 5 docs (4 standalone + 1 transitive), 1
script, 1 test method, ~67 lines of build_journey() code, the
JOURNEY_STYLE constant, 8 entries from the prototyping index list,
1 outdated lesson bullet.

Author: claude