thorvg 0.4.2

Documentation-only release. No code, API, or behavior change — a patch
bump so the revised docs render on docs.rs.

Documentation

• Reviewed every public item against the ThorVG C API (thorvg_capi.h
  contracts and tvgCapi.cpp behavior) and corrected inaccuracies in
  error conditions, parameter units/ranges, ownership/lifetime claims,
  defaults, and experimental status. Notable fixes: TextMetrics::advance
  is the vertical line advance (not a horizontal run advance); the Scene
  drop-shadow default direction claim was removed; Shape stroke defaults,
  the empty-dash-slice error, and circular trim-path wrapping are now
  documented.
• Normalized the rustdoc to one style — terse summaries, # Errors /
  # Safety / # Panics sections, intra-doc links, sharpened safety and
  reliability contracts, and a per-module link to the upstream
  C API docs.