feat(render): book-style page-numbered [TOC] via BookTocRenderer

[DemchaAV]

Stacked on #40 → #39 — merge order: #39 (split) → #40 (footnote fix) → this.

Why

Tier 2 flagship of the post-0.2.0 roadmap: the [TOC] link list is right for screen, but a print deliverable wants a real contents page — dot leaders and page numbers. The engine's 1.9 addTableOfContents/TocBuilder resolves page numbers automatically (second layout pass); the library only had to feed it label+anchor pairs it already plans.

What changed

• BookTocRenderer (new public renderer): opt in via the theme's own extension seam —
  MarkdownTheme.builder(base).renderer(TocNode.class, new BookTocRenderer("Contents")) — zero
  new tokens/flags. Dot leaders + muted page numbers + optional title row; labels indent by
  heading level using non-breaking spaces (the code-block idiom; plain leading spaces are
  collapsed by text layout and base-14 fonts have no em-space glyph). Entries stay clickable
  GoTo jumps; empty/no-heading documents degrade exactly like the default TocRenderer, which
  remains the default.
• Runnable BookTocExample (paired with the page-number footer — the classic report look);
  README snippet + CHANGELOG.

Verification

./mvnw clean verify javadoc:javadoc → BUILD SUCCESS, 169 tests, 0 failures. BookTocTest:
a digit-free multi-page document proves the resolved page number is printed on the contents
page and entries emit GoTo; a no-heading [TOC] renders nothing. Visually confirmed:
"Contents" + Introduction/Getting started/Configuration (indented)/Appendix with dot leaders and
live page numbers 1/2/2/3.

[DemchaAV]

Blind cold review verdict: approve with follow-ups — both landed in 7daa835: (1) the nesting-indent path is now covered (h2 indent asserted positionally — PDFBox extracts the NBSP indent as leading blanks — plus h2-only minLevel normalization and the page number tied to its own entry's line); (2) class Javadoc says non-breaking spaces. The label re-parse edge (engine markdown=true re-parses TocBuilder plain-text labels; # Rating \*hot\* would italicize in the book TOC) is recorded as a tracked audit in the private roadmap — a session-wide markdown(false) decision is deliberately its own PR since several p.text(...) sites share the latent surface. Suite green: 171 on this branch, 175 at the top of the stack (#42 rebased).