feat(tour): unify index Topics list — parts + docs, muted summaries

Collapses the index.html TOC from two separate cards ("Parts" and
"Topics") into a single unified list under one "Topics" header.
Each row has the same shape: a bright title link and a muted
one-liner context (section count for parts, summary for docs) —
matching the dek-line treatment on the socket.dev footer reference.

Build-side:
- scripts/tour.mts: new rewriteIndexContents replaces meander's
  Parts card + any legacy Topics card with the unified block.
  Uses node-html-parser (newly pinned devDep) instead of regex —
  selector queries + element replacement survive future meander
  output whitespace changes without brittle patterns.
- Removes the now-redundant Index-page TOC post-process step
  (<ul> → <ol class="wt-parts-ol"> + "Part N:" prefix stripping)
  since rewriteIndexContents replaces the whole card anyway.

Style-side:
- walkthrough-overrides.css: replaces the old .wt-parts-ol rules
  with .wt-contents + .wt-contents-row + .wt-contents-title +
  .wt-contents-summary. Muted summary color (var(--eyebrow)), tight
  but not crowded row rhythm (8px padding + hairline separators),
  hover tint on titles.

devDep: node-html-parser 7.1.0 (exact pin, not catalog — this repo
has no catalog and adding infrastructure for a single helper was
disproportionate).

Author: jdalton

package.json

@@ -90,6 +90,7 @@
     "magic-string": "0.30.21",
     "marked": "18.0.0",
     "nock": "14.0.10",
+    "node-html-parser": "7.1.0",
     "oxfmt": "0.37.0",
     "oxlint": "1.52.0",
     "pacote": "21.0.1",