feat: add result-bearing 1D NMR integral curves

[nmrtist]

Context

1D NMR integrals were primarily represented by editable rectangular regions.
The normal figure view did not provide cumulative integral curves, and the
result-bearing representation was not shared across screen and export
renderers.

Reference normalization was also fixed to 1.0 for 1D integrals and used a
different state model from 2D integrals.

Changes

• Add lightweight integral-curve descriptions to the shared Figure model.
• Derive signed cumulative curves from the source spectrum using
  spacing-aware trapezoidal integration.
• Share curve geometry and label layout across screen, SVG, PDF, bitmap, EMF,
  and clipboard vector rendering.
• Use compact vertical labels on the right side of each curve with overlap
  mitigation and viewport clamping.
• Keep integration rectangles and handles as editing-only UI chrome.
• Support arbitrary normalization reference values for both 1D and 2D NMR.
• Remove reference-specific curve colors and plot markers.
• Synchronize integral curves across apply, undo, redo, live editing,
  cancellation, figure rebuilds, and processing changes.
• Respect primary-series visibility and prevent cross-plot hover activation.
• Handle zero, near-cancelling, non-finite, and undersampled integrals without
  producing invalid coordinates.
• Avoid repeated sorting for monotonic NMR spectra while retaining a fallback
  for non-monotonic series.
• Add serde defaults for additive integral and figure fields without
  reintroducing the previous dual reference model.
• Update the English and Simplified Chinese integration documentation.

Testing

• cargo pr-check — passed all 7 checks
• npm run build in docs/ — 63 pages built successfully