fix: table parser honors escaped pipes; JS renderMarkdown matches heading offset

[propcgamer20-png]

Two build_explainers.py <-> explainers-ui.js parity bugs.

#552 - table parser breaks on a literal pipe in a cell (2 pages already misrendered)

scripts/build_explainers.py's split_row and assets/explainers-ui.js's parseTable both did row.split("|"), with no support for GFM's \| escape. A literal pipe inside a cell started a spurious column:

• explainers/reject-inference.html - the IPW row (w(X) = P(S = 1 | X)) rendered 6 <td> for a 4-column table, misaligning every later column
• explainers/base-rate-fallacy.html - PPV (P(Y = 1 | Ŷ = 1)) split into two separate <th>

Both parsers now split on (?<!\)\| (unescaped pipe only) and unescape \| -> | in each cell. reject-inference.md's two bare pipes are escaped as \| in source (base-rate-fallacy.md already had them escaped - the parser just wasn't honoring it). Pages regenerated; the IPW row is now 4 <td>, the PPV header one <th>.

#553 - explainers-ui.js renderMarkdown drifted out of heading-offset parity

A prior fix offset build_explainers.py's markdown headings by +1 (the page hero already renders a real <h1>). explainers-ui.js's renderMarkdown was never updated - it still emitted # X -> <h1>. It now applies Math.min(level + 1, 6), matching Python (# X -> <h2>, ###### X -> <h6>). This path isn't reached in production today (no explainer page loads explainers-ui.js), but a later "fix" assuming parity would have reintroduced the duplicate-<h1> bug.

Tests

• test_parse_table_honors_escaped_pipe_inside_a_cell
• test_render_markdown_offsets_heading_levels_by_one
• test_explainers_ui_js_stays_in_render_parity_with_build_explainers - source-level guard that both fixes are mirrored in the JS

pytest tests/test_build_explainers.py -> 7 passed. check_em_dash / check_broken_links / check_generated_files_current pass. ruff clean.

Closes #552
Closes #553

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