Honor expandChorusDirective in PdfFormatter

[martijnversluis]

Fixes #2240

The PDF layout engine always read the song's bodyParagraphs, so {chorus} directives were never expanded in PDF output — unlike TextFormatter and HtmlFormatter, which honor expandChorusDirective.

Changes

• LayoutEngine now resolves the paragraphs to lay out from the config: it uses expandedBodyParagraphs when expandChorusDirective is enabled, and returns the resolved array instead of writing it back onto the song.
• expandChorusDirective is added to LayoutConfig and passed through from PdfFormatter.
• Dropped the redundant song.clone() in paragraph selection: the source paragraphs are never mutated (all pushed paragraphs are clones), and cloning stripped line numbers, which LineExpander needs to expand {chorus}.
• Song#renderParagraphs is no longer used internally and is deprecated.

Note on repeatedSections

The PDF default repeatedSections: 'title_only' collapses repeated sections to their title. Since an expanded {chorus} reproduces the chorus, it is treated as a repeat and collapsed to its title under the default config. To get output identical to text/HTML (the expanded chorus rendered in full), combine expandChorusDirective: true with a non-collapsing repeatedSections such as 'full'.

Tests

Added PDF formatter tests asserting chorus lyrics are laid out twice when expandChorusDirective: true and once when false (with repeatedSections: 'full').

[isaiahdahl]

Noticing the fact that repeatedSections needs to be set to full for this to work. So I dug around on that.

1. expandChorusDirective recalls the chorus.
2. repeatedSections then processes the recalled section.
3. PDF defaults repeatedSections to title_only.

As a result, {chorus} is expanded, but the default PDF configuration can remove its lyrics and retain only its label. hide can remove the recalled section. full retains the full section.

Labels also affect the result. A bare {chorus} can match the original chorus label and be reduced as a repeat. {chorus: Sing it} can use a different label and avoid that match. The same configuration can therefore produce different output based on the directive label.

I see three possible paths:

1. Keep the current processing order. Treat repeatedSections as the final display rule. Add a warning and document the interaction.
2. Make expandChorusDirective: true always render the full recalled chorus. This requires cloned lines and source tracking so repeat processing can skip generated sections.
3. Add a separate chorus-directive display option. This is a larger configuration and migration change.

I prefer path 1 for this PR because it keeps the fix small. I suggest these amendments:

• Pass expandChorusDirective through MeasuredHtmlFormatter as well. It uses the same layout engine.

• Use the shared warn() helper when expandChorusDirective is true and repeatedSections is not full.

• Use wording such as:

  │ expandChorusDirective runs before repeatedSections. The current repeated-section mode can hide part or all of the recalled chorus. Use repeatedSections:
  │ "full" to show the full chorus.

Warn only in this case:

  expandChorusDirective === true &&
  repeatedSections !== 'full'

This would make the current behavior explicit without expanding this bug fix into a larger API redesign, since we kinda have overlapping responsibility in the config with repeatedSections and expandChorusDirective