Mark positioning, script-aware GPOS/GSUB feature lookup, and ligature feature-tag support (#2503)

[swmal]

Summary

Renders mark-to-base positioning, makes GPOS/GSUB feature lookup
script-aware, and exposes ligature feature selection through
PdfPageSettings. Closes #2503.

Changes

• Mark positioning: MarkToBaseProvider now computes pen-relative,
  additive offsets (HarfBuzz convention); PdfContentStream reads
  XOffset/YOffset via Ts and paired TJ adjustments.
• Script-aware feature lookup: new ScriptFeatureResolver filters
  FeatureList entries by ScriptList/LangSys instead of tag alone.
  Fixes a real defect where an arab-only kern record leaked into
  Latin text. Applied across all six GPOS/GSUB processors (kerning,
  single adjustment, mark-to-base, ligatures, chaining contextual,
  single substitution).
• Ligature feature tags: LigatureProcessor now reads tags from
  GsubFeatures instead of a hardcoded "liga", and unwraps
  extension-wrapped (Type 7) ligature lookups, which GSUB does not
  flatten the way GPOS does.
• Public API: GsubFeature/GposFeature flag enums and
  PdfPageSettings.GsubFeatures/GposFeatures, wired through
  PdfTextShaper.

Verification

Confirmed in exported PDFs: Á now aligns with its precomposed
control glyph, a previously leaking period no longer receives a
Ts offset, and EBGaramond's dlig merges "Th" into a single,
measurably narrower ligature glyph when requested. Extensive unit
test coverage, including synthetic-table regression tests for the
script-leak defect.

Known gaps

• Measurement-path/render-path width parity is still unverified.
• No dedicated script-leak test for three of six GPOS/GSUB processors
  (same fix, same pattern as the tested ones).